What is it that makes Birthday Cards so dang successful? Back in September, when looked at why birthday messages are popular on Facebook, the RockYou app only had two million users. Now it has over 40 million monthly active users (MAU), and regularly appears at the top of this AppData list of top Facebook apps by MAU growth.

Our answer then was that public wall postings are valuable to both sender and receiver on Facebook in a dynamic that beats sending a Hallmark card. But there’s more to Cards — after all, it beat out its larger competitors, some of whom were ahead at one point.

RockYou has pulled a couple clever tricks with Cards, like cross-promoting the app with its game Zoo World. But we’ve got another guess at the developer’s secret sauce: they’ve built a genuinely good app. Take a look sometime. Building a custom card is actually fun, and the app also comes with features like virtual currency, gifts and a calendar to track significant occasions.

Birthday Cards is by far the biggest gainer on the list this week, but a few others are doing quite well, too. Take a look:

	Name	MAU	Gain	Gain, %
1.	Birthday Cards	41,242,773	+6,009,722	+14.57
2.	Calendario de Amigos	4,586,492	+3,081,641	+67.19
3.	Static FBML	12,895,673	+1,906,921	+14.79
4.	FarmVille	75,476,475	+1,120,699	+1.48
5.	Facebook for iPhone	26,053,163	+924,310	+3.55
6.	Happy Island	10,612,387	+892,445	+8.41
7.	Tiki Farm	6,333,103	+801,700	+12.66
8.	FishVille	25,221,818	+705,243	+2.80
9.	Mafia Wars	24,398,295	+591,077	+2.42
10.	Gangster City	706,173	+568,997	+80.57
11.	Yearbook	4,786,332	+563,976	+11.78
12.	Little Warrior	1,057,079	+530,889	+50.22
13.	Wild Ones	2,197,693	+523,510	+23.82
14.	MindJolt Games	16,109,140	+472,473	+2.93
15.	Texas HoldEm Poker	25,066,873	+464,276	+1.85
16.	Graffiti	5,910,433	+460,251	+7.79
17.	Facebook for BlackBerry® smartphones	13,517,451	+423,138	+3.13
18.	Music	11,831,851	+417,122	+3.53
19.	My Town	1,551,376	+390,730	+25.19
20.	Country Life	8,017,883	+379,550	+4.7

Skipping Calendario de Amigos, which is successful for some of the same reasons as Birthday Cards, take a look at Static FBML. This is the second week in a row that it has come in at number three, although this week it added another 500,000 users over its previous gain. The growth is a bit curious, since the app, used to create customized boxes within profile pages, requires some ability with HMTL or Facebook’s own markup language — in fact some, big app developers use it, we hear.

There probably aren’t really 12 million plus HTML experts on Facebook — one hint is that Static FBML’s daily active user base is much smaller, at just over 700,000 people. Also, there seems to be a high proportion of Turkish-language users on the app. It’s possible there’s a trend among that user-group of installing Static FBML to make minor adjustments on their profiles.

Farmville is next, but we cover its ilk over at Inside Social Games. We’ll wrap up with number five: Facebook for iPhone. This app and Facebook for BlackBerry® smartphones mysteriously vanished last week when their new user growth dipped in unison; see below. In general, mobile apps should be growing for some time as ever more people pick up smartphones.