This past week was typical, in terms of which Facebook applications we saw gain the most users. Social games of various genres continued to gain millions of users. Similar to past weeks and months, Zynga’s virtual farming game FarmVille grew by 5.11 million monthly active users to reach 46.0 million, and the company’s role-playing mobster game Mafia Wars grew by 2.05 million to reach 23.2 million. They came in at number two and number four on our top 20 list.

However, number one goes to RockYou’s Birthday Cards, a full-featured virtual card-giving application. It was gradually declining over the last few months, started growing at the end of August, and reached around nine million users as of the first week in this month. Since then, it has for whatever reason found the curve in the hockey stick of growth: The app has gained 5.23 million MAUs in just the past seven days to reach 16.3 million.

		Name	MAU	Gain	Gain, %	Developer
1.		Birthday Cards	16,273,979	+5,276,859	+48.0	RockYou!
2.		FarmVille	46,048,437	+5,111,332	+12.5	Zynga
3.		Your Indian Name!	4,132,326	+4,132,058	+1 541 810.0	AppSphere Inc.
4.		Mafia Wars	24,239,040	+2,051,778	+9.3	Zynga
5.		My Fishbowl	3,302,112	+1,568,713	+90.5	TwoFishes Interactive
6.		Causes	30,188,784	+1,459,385	+5.1	Causes
7.		Restaurant City	14,359,677	+1,291,766	+9.9	Playfish
8.		Country Story	6,207,442	+871,019	+16.3	Playfish
9.		YoVille	16,291,957	+769,233	+5.0	Zynga
10.		Pet Society	17,669,556	+705,753	+4.2	Playfish
11.		Horoscopes	3,084,299	+676,227	+28.1	RockYou!
12.		Music	10,338,786	+649,598	+6.7	iLike, inc
13.		Texas HoldEm Poker	17,199,654	+578,952	+3.5	Zynga
14.		(Lil) Farm Life	3,132,399	+558,930	+21.7	Greenpatch Inc
15.		What Type of Wife are you or will you be?	894,967	+540,820	+152.7	—
16.		Tattoodle	5,032,939	+535,049	+11.9	Make The Web Better
17.		Animal Paradise	2,043,280	+523,034	+34.4	Rekoo
18.		What’s your Actual Age?	1,220,122	+492,216	+67.6	q
19.		Kad Raya	504,166	+431,307	+592.0	—
20.		Give Hearts	6,932,658	+429,392	+6.6	6 waves

Number four, Your Indian Name!, has seen even sharper growth, and quite abruptly at that. The app translates your name into Hindi, and lets you very easily post that information to your Facebook wall. It grew from 265 users to 4.13 million by the weekend (there may be a problem with the data here). The app’s creator, AppSphere Inc, has 57 total applications by our count.

Seven apps in total gained more than a million users. At number five was My Fishbowl, made by TwoFishes Interactive. The virtual aquatic life game had been slowly growing from nothing in early August, until September 15 when it hit its stride, er, stroke. It grew by 1.57 million to reach 3.30 million MAUs. Keep an eye on this one.

At number six was Causes, one of the only non-game apps on the list. It grew by 1.50 million to reach a stately 30.2 million MAUs. At number seven was Playfish’s Restaurant City, the virtual restaurant game grew 1.29 million to reach 14.4 million.