Online dating has been getting more and more mainstream over the past few years and, unsurprisingly, many companies have been trying to provide versions on Facebook as well. The promise is that developers can use Facebook’s social graph and communication channels to build applications match the right people up. In the course of our research, we found that the most popular services are using applications on Facebook to help people find love, although there are others that are Page-centric, bypassing apps all together.

Yet, there doesn’t appear to be a clear leader among dating services. Zoosk is historically the biggest, and although its monthly active user count has been declining lately on Facebook, it also has built a stand-alone web site and a presence elsewhere on the web. The company, which raised lots of cash last year, strikes a good balance between providing free services like search versus paid ones like chat.

The bottom line, from our review, is that Facebook-focused dating apps like Zoosk and Are YOU Interested? performed the best in terms of number of monthly active users and technical set-up with their apps, and in terms of number of fans on their Pages. More “traditional” online services that set up Pages on Facebook, such as eHarmony and Match.com, integrated parts of their web site services into Facebook but didn’t seem to be setting up shop on the social network specifically to grow or make money. For example, eHarmony didn’t offer anything on its Facebook Page other than links to its site and dating advice and even though Match.com did include an app on its Page, the content seemed to come from its web site.

Facebook Connect featured prominently in a few of these services, too. Thread (formerly Frintro) lets people sign in to its site using Facebook Connect, and then allow people to anonymously tell Facebook friends-of-friends if they’re intereste. Are YOU Interested? has a Facebook Connect integration that’s similar, even importing the profile information of Facebook friends who match your sexual orientation and asking whether you’d like to let them know you’re interested via the service. Finally, Zoosk uses Facebook Connect, too, but it just jumps from the site back to your Facebook Zoosk profile.

Searching for “dating” on Facebook will yield hundreds of Pages, apps and groups. Many of these are small and explicitly specialized, such as big women, Latinos or Blacks, interracial dating, lesbians or gays, dating for specific states in the U.S., dating by religion like Catholic or Jewish, cougar dating or conservative/liberal dating. Most of these don’t include apps, but rather, are gathering places for like-minded people looking to date and as a consequence of not including an app, feature many fan uploaded photos. While Zoosk and other large apps had hundreds of thousands of fans and monthly actives, there were dozens of smaller apps that had anywhere from a dozen to a few thousand users.

Turning first to the classic online dating services eHarmony and Match.com first, we see that, although these services have been very successful online and we’ve seen them previously advertise on Facebook, their transition to social network dating hasn’t been seamless or complete. As previously mentioned, eHarmony with about 4,800 fans, links mostly to its web site and includes a tab with dating advice but has no specific Facebook integrations.

Match.com has just about 1,800 fans and includes a Success@Match tab with success stories of the service and a Matchmaker tab, which includes an app that Facebook users can utilize to find matches for their Facebook friends. The app is attractive and easy-to-use. After allowing access, the app sets up a slideshow of your Facebook friends, one list for men another for women. Upon choosing a friend, you select whether they are interested in men or women and their location, then the app displays results for your friend, you can select five, send those to your Facebook friend, and then of course, publish the fact to your Wall.

We also looked at Zoosk, Are YOU Interested? and Be Naughty! to see what big Facebook dating apps were doing. First off, all three seemed to have a combination of free and paid content available to Facebook users and incorporated all activities into Facebook in different ways.

Are YOU Interested? with more than 1.2 million monthly active users is an app that requires the user to enter basic information such as age, location and sexual orientation, then it finds you some matches near your area listed by first name. Users can then view more photos of their matches, their app profile and send a gift or message within the app (as opposed to on Facebook); you can also see if people are interested in you, have sent you messages, etc. There’s also an upgrade option, which gives you access to more content such as seeing your friends’ matches, that costs $9.99 a month for six months, $12.99 a month for three months and $19.99 for one month.

Be Naughty! is an app with more than 976,700 monthlies and also allows you to see matches by age and location, although a lot of comments in the discussion section bemoaned the fact that you cannot chat with any of them unless you pay. This app doesn’t allow users to see much content unless you update your app profile and has some key Facebook integrations that seem designed to keep users coming back and using the app as much as possible. Unless you want to pay to use the site, you have to use it a lot and get friends to use it to earn points to unlock a chat session with a match for free, which goes for 800 points. Users can earn these by uploading photos (50), becoming a fan (50), completing your profile (100) and inviting friends (100), so you can see the potential for users inviting all of their friends to join.

Finally, Zoosk towered above the other dating Pages and apps with almost 3.7 million monthly active users and almost 891,000 fans. Zoosk users can fill in their app profile (“date card”) and also search for matches near to their location by age and sexual orientation.

Users can “friend” other app users, send others winks and emails in the app and see who’s been viewing their profiles. Zoosk has a Messenger app that users can download to chat with other Zoosk users and there’s also an activity feed in the app. As with the other apps, Zoosk offers more for people who pay $9.99 for 12 months, $19.95 for three or $29.95 for one month.

We previously wrote about the possibilities for using Facebook dating with Facebook Connect and, slightly less than a year later, we see that the web sites that used it weren’t necessarily the ones to pull ahead. Rather, like with many other things we’ve seen on Facebook, it’s the services that adopt their model to the way the social network is being used that have been the most successful.