Earlier today, Facebook’s Director of Engineering Aditya Agarwal announced the launch of Facebook Prototypes at TechCrunch50 in San Francisco. The apps, all built by Facebook engineers during recent company hackathons, “let us share the experimentation going on inside the walls of Facebook with the rest of the world,” according to Facebook engineer Lee Byron.

We sat down with Agarwal and Product Manager Ruchi Sangvi this afternoon to talk about at the prototype apps Facebook released.

1. Recent Comments Filter

Developed by Dan Schaffer (intern)

The Recent Comments Filter app is one of the most useful of them all. When users add the app, a new “Comments” filter appears on the News Feed that shows the most recent posts that have been commented on by your friends, with your friends’ comments highlighted. It’s a powerful way to see what your friends are engaging with across Facebook.

2. Similar Posts

Developed by Raylene Young (and others)

Like the Comments filter, the Similar Posts app adds more productivity to filtering the News Feed. When you add the app, a new “Similar Posts >” action item appears next to items in the News Feed, that when clicked automatically shows all other related posts in your stream. The app automatically selects important keywords and finds other stories that match the selected story in your News Feed. While the matching algorithm is somewhat rudimentary, it’s a fantastic way to see which of your friends are talking about the same thing.

3. Photo Tag Search

Developed by Al Urim (intern)

Photo Tag Search is a handy application that allows you to easily find photos tagged with multiple friends. For example, say you wanted to find photos of your friends Liz and Jessica, but didn’t want to flip through Liz’s 1,000 Facebook photos for ones that Jessica is also in. Now, you can do a tag search from the Facebook Photos main page and find photos easily.

4. Enhanced Event Emails

Developed by Paul McDonald

Enhanced Event Emails is also a very handy application that automatically attaches iCal files to your Facebook Event invitation emails. This means it’s easy to integrate Facebook Events with whatever calendaring product you already use, like Google Calendar, Outlook, and others.

5. Desktop Notifications for Mac

Developed by Lee Byron

Initially released last week, Desktop Notifications for Mac is a simple utility that displays incoming Facebook notifications and Facebook messages in real time on your desktop. With Desktop Notifications for Mac, you won’t miss anything that’s happening on Facebook whenever you’re using your comptuer.

Facebook has just announced it has reached 300 million monthly active users around the world. The company has added 50 million users in just the 75 days, that’s 670,000 users per day. In mid-July, Facebook announced that it had reached 250 million.

Facebook’s global audience has doubled since the beginning of the year, adding 150 million new users since January. Notably, over 71% of Facebook’s userbase resides outside the United States – as of today, the company reports 85 million active American users.

Perhaps even more impressive is that the company also says it is now “free cash flow positive.” From the company blog post on the news, from founder Mark Zuckerberg:

We’re also succeeding at building Facebook in a sustainable way. Earlier this year, we said we expected to be cash flow positive sometime in 2010, and I’m pleased to share that we achieved this milestone last quarter. This is important to us because it sets Facebook up to be a strong independent service for the long term.

For those not familiar, “free cash flow” means the amount of cash that a company generates after budgeting costs required to maintain and expand itself.

Earlier this year, Facebook CEO Mark Zuckerberg said that the company is expecting to see “70% growth in revenue year over year” in 2009 and that Facebook will be “cash flow positive in 2010.″ Facebook board member Marc Andreessen also recently said Facebook would do $500 million in revenues in 2009, up from an estimated $280-$300 million in 2008.

What’s driving Facebook’s revenue growth? A combination of revenue streams: Facebook’s self-service ad business has been very strong lately, it continues to invest heavily in brand advertising efforts, and it also continues to release many experimental expansions to its virtual goods and virtual currency business, Facebook Credits. The company is also still deriving revenue from its advertising deal with Microsoft, signed when Microsoft invested in Facebook in 2007.

Update: So what sort of revenue growth has Facebook actually seen? We asked company executive Elliot Schrage: He couldn’t comment on what exactly was growing revenue, nor could he comment on the company’s overall financial numbers today. We don’t expect the company to say much more this year.

Update: In fact, Facebook has just launched Prototypes. It’s not live for everyone yet (including us), but once it is, you’ll be able to see it as an option in the left-hand navigation menu in the application directory. As we wrote below, Prototypes is a way to find experimental projects built by solo Facebook engineers.

Facebook’s engineering team has a tradition of hackathons — everyone hangs out together and builds experimental features for the site. Those features are sometimes live. And we may have a much easier way to find them, via a new directory that appears close to launch, called “Prototypes.”

The first screenshot about the section appeared last week. But the actual directory wasn’t live. Today, though, Facebook’s official Twitter account accidentally tweeted about the feature, and screenshots escaped into the wild. It looks like there are some really useful in-house apps coming. One is a desktop notifications app, another is a way to find similar posts from your homepage, and the one I want the most is a way to integrate iCal notifications into email.

Facebook has a developer garage planned today at the TechCrunch50 conference. Perhaps we will have an update about these prototypes later today?

While Zynga has partnered with a variety of offer monetization platforms over the years, we’ve been hearing that the company has been increasingly driving some users to an offer network it may either own or be strongly allied with. Now, we know the name of that service – DoubleDing.

While we haven’t been able to confirm if DoubleDing is either owned by Zynga or is a separate entity controlled by the company, its executives, or investors – Zynga has not responded to our request for comment yet – we do believe that it’s part of Zynga’s strategy to bring a greater part of the monetization value chain closer to being in-house.

Why would Zynga want to do so? Two reasons…

> Continue reading at Inside Social Games

Wispa, an aerated chocolate bar distributed by Cadbury in the United Kingdom, has been getting a lot of new fans, as we noted yesterday. Looking closer, the reason appears to bode well for a dedicated group of people who love a type of Wispa bar called “Gold” because of its extra caramel.

The Gold, along with the rest of the Wispa brand, was removed from distribution in 2003 as part of a company effort to streamline its product lines. A Facebook-fueled campaign in 2007 convinced the company to bring back the original Wispa bar, first as a limited edition. It did so well that the company made it permanent — and is now trying to see how the Gold does on the market. A group of Gold devotees, only 22,000 strong, convinced the company to re-introduce it earlier this year.

The Gold was just launched on September 8th, and since then the page has grown from a flat-lining 266,000 fans to nearly 500,000 today. It’s not clear what’s causing the growth. Cadbury has a contest for Gold, where you can compete to write a message that will appear on billboards near where a loved one lives in the country. But the contest is run off of Cadbury’s home site, and doesn’t appear to even mention Facebook.

One possibility: The hardcore group of Gold lovers is the visible fraction of a far larger nugget of fans, people who are showing up now that the bar is out on shelves. Although these fans don’t seem to know about the actual Wispa Gold page, which is basically nonexistent. If the Wispa page keeps growing like this — and if it’s growing because of Gold’s reintroduction — Cadbury might want to make this version of the Wispa bar permanent as well.

Several weeks ago, Facebook launched a new “Fan Box” widget, allowing Page owners to add a widget to any website to turn visitors into Facebook fans. Since then, many Page owners have done just that, and are finding the Fan Box widget an effective way to grow their fan base.

However, as you may expect, some Page owners have been putting the Fan Box widget not only on their own websites, but also inside banner ads on other websites, encouraging ad viewers to fan the page in line through the Fan Box widget in the ad.

For example, Spongecell has created a new line of ads (see examples for Olive Garden, Jiffy Lube, and General Mills) that incorporate the Fan Box widget, and Spongecell CSO Marc Guildimann says conversion rates have been high – almost as many ad viewers are becoming a fan as clicking through to the advertiser’s landing Page in trials so far.

However, it appears that Facebook is not terribly keen on new ad units like Spongecell’s that enable people to become a fan inside the ad. In fact, Facebook’s terms of service actually prohibit using the Fan Box widget in any ad at all. Section 6.12 reads, “You may not place a Fan Box widget in an advertisement.” Why?

According to a Facebook spokesperson, “We want Page owners to have an easy way to connect with fans both on and off of Facebook.  In order to protect the the Fan Box widget from being used for the wrong reasons, we do not allow it to be used in third party advertising.”

While Facebook wouldn’t go into more detail, it’s safe to assume that Facebook wants to protect the “Become a Fan” experience from becoming too intertwined with aggressive online ads that it hasn’t approved. One can imagine the variety of ways advertisers could (potentially misleadingly) push users to become a fan in an ad unit on a web site, then pollute their Facebook stream later. Facebook wants more control over that experience, even if it means partially restricting growth for Facebook Pages.

However, it also means that high-quality ads, like those from Spongecell, are not going to become more common. Rather than having an outright prohibition on ads like these, we think it would make more sense for Facebook to allow trusted Page owners to drive more ad viewers to convert to Page fans in ads across the web – but Facebook doesn’t plan on allowing anyone to do so for now.

While many Page managers religiously check Facebook’s Insights dashboard to get the latest stats on traffic and engagement, some encountered news alerts instead of their normal Page stats today. According to the alert messages, Page stats are said to be unavailable due to an insufficient “number of fans interacting with your page” – despite the fact that some Pages in question had large audiences.

Facebook confirmed to us this evening that the Insights issues today were indeed a bug. “Pages are very important to us and we’re working on fixing this issue as we speak,” a Facebook spokesperson tells us. “None of the data will be lost and insights will appear as they had previously.”

At first, I wondered if Facebook’s new @ status tagging method was too arcane for the typical user. The idea of putting an @ symbol in front of somebody’s to help get their attention online, well, might not be intuitive. Especially considering that many Facebook users aren’t familiar with how microblogging service Twitter works.

But as Twitter has shown, if there’s effective self-promotion involved with @, people will use it, especially celebrities. Today, Facebook rolled out the @ feature to all of its nearly 300 million users, and big names jumped aboard. Two well-known bands that I am neither a fan of on their Facebook pages nor in real live, The Jonas Brothers and Linkin Park, posted about it.

And, less surprisingly, Facebook marketing lead Randi Zuckerberg was interviewed about it on CNN Live. That’s three mainstream hits for a product that a few years ago would have been unknown to most people.

Oh, and in case you’re still wondering what I’m talking about, here’s how the @ feature works. You type in @ within the status update box on Facebook. Then, start typing the name of friends, Pages, groups, and events you’re a part of. You’ll see a drop-down menu of options. Choose one, and that person’s or organization’s name will appear once you hit publish. But the @ feature is actually a tag, sort of like how you tag photos of people on Facebook — or actually, almost exactly like you direct a message at another Twitter user. Once tagged, the recipient will receive a notification email and the tagged item will show up on their wall.

This is sort of like how Twitter has worked for years, with the crucial difference being that the default was for the message, and the Twitterer’s profile, to be public. This has allowed messages to spread and be searched for easily. Indeed, Facebook itself used Twitter to share many of the examples above.

Still, with free marketing like what these celebrities are providing, Facebook’s status tags are getting a boost that could overcome the hindrances of being private — and they could get more of one assuming these celebrities and others start using them regularly. Use of @ is likely to grow fast on the site.

Last week, beverage maker Vitaminwater launched a new campaign on its Facebook page: An in-house application that includes games and a contest where users can help choose a new flavor of its vitamin sugar water. The result, so far? The page was the fastest-growing among any major brand over the week, according to our PageData analytics service. It gained 323,000 fans to reach 767,000 people today.

More generally, here are a few other notes about the top leaders this week. Coming after Vitaminwater was Cadbury Wispa, a relaunched line of candy bar dearly loved in some parts of the world. Its page gained 195,000 fans to reach 461,000 today. Behind it, a Venezuelan opposition politician named Leopoldo Lopez saw a massive increase, growing 189,000 fans to reach 206,000.

And a quick note. At the top of list is a page called Sarcasm Society. It is apparently the page for a site of the same name. Sarcasmsociety.com comes up as a top result when you google “sarcasm,” yet it has almost no actual traffic, according to Compete. And, finally, almost all of the page’s growth has happened within the span of a day or two. So we’re assuming there’s something fishy here. Same for another page on the list, called Getting Paid.

	Name	Fans	Gain	Gain, %
1.	Sarcasm Society	1,453,401	+1,448,849	+31,828.84
2.	vitaminwater	766,901	+323,389	+72.92
3.	Cadbury Wispa	460,575	+195,395	+73.68
4.	Leopoldo Lopez	205,820	+189,214	+1,139.43
5.	Mafia Wars	4,137,663	+155,338	+3.90
6.	Cheryl Cole	363,511	+116,475	+47.15
7.	Getting Paid	1,836,626	+97,148	+5.58
8.	Texas Hold’em Poker	3,040,150	+92,558	+3.14
9.	Vin Diesel	6,224,611	+83,452	+1.36
10.	Best Buy	829,987	+81,913	+10.95
11.	Facebook	5,093,035	+74,450	+1.48
12.	iTunes	1,922,436	+73,712	+3.99
13.	Total Wipeout	308,774	+68,340	+28.42
14.	Selena Gomez	3,001,306	+66,664	+2.27
15.	House	2,148,933	+65,713	+3.15
16.	David Guetta	1,085,442	+64,549	+6.32
17.	The Bible	487,338	+63,510	+14.98
18.	Roger Federer	2,979,384	+63,282	+2.17
19.	Glee	232,765	+62,650	+36.83
20.	Celebs on Facebook	1,298,590	+61,222	+4.95

Earlier today, Facebook released its new “status tagging” feature to all users, a new way to tag friends, Pages, groups, and events in status updates much like mentions on @twitter work.

However, while status tagging is a powerful tool for users and Page administrators, Facebook hasn’t officially launched any new API’s for developers to take advantage of the feature. Last week, Andrew Huang, the product manager at Facebook responsible for status tagging, told us that while Facebook waiting to see how the feature plays out before they expand it, Facebook is thinking about how new Platform APIs could enable users to tag Facebook connections.

Nevertheless, some developers are finding at least one way to tag friends in status updates from apps. All you have to do is use the following format in status updates, and Facebook will automatically display status tags:

Tagging @[USERID:blah] in a status update from my app

Where “USERID” is the numeric Facebook ID of the user you want to tag, and “blah” can be any string (Facebook will always print and link the name of the tagged user no matter what you put here, but you must enter some text here in order for this to work). You can also simply type this syntax in the regular Facebook publisher, and it will work.

Thanks to Tom Waddington for the tip.