Happy days continue for Birthday Cards and Zoo World, the two biggest Facebook apps from RockYou, on this week’s list of fastest-growing games by monthly average users (MAU), from AppData. The pair seems almost unstoppable, with Zoo World, the more game-like of the two, potentially ready to break 20 million users next week.

Here’s the full list:

	Name	MAU	Gain	Gain, %
1.	Birthday Cards	45,831,329	+4,588,556	+10.01
2.	Zoo World	18,008,311	+1,802,177	+10.01
3.	FarmVille	76,677,249	+1,200,774	+1.57
4.	Truth about friends	590,265	+590,117	+99.97
5.	Happy Island	11,146,511	+534,124	+4.79
6.	Gangster City	1,233,329	+527,156	+42.74
7.	My City Life	798,564	+520,032	+65.12
8.	Little Warrior	1,497,204	+440,125	+29.40
9.	My Town	1,961,793	+410,417	+20.92
10.	Wild Ones	2,603,987	+406,294	+15.60
11.	MindJolt Games	16,454,075	+344,935	+2.10
12.	Go to Hell	347,382	+281,970	+81.17
13.	Mafia Wars	24,650,755	+252,460	+1.02
14.	Texas HoldEm Poker	25,306,358	+239,485	+0.95
15.	開心寶貝	1,355,142	+196,668	+14.51
16.	PetVille	18,133,636	+181,872	+1.00
17.	Band of Heroes	1,158,846	+170,070	+14.68
18.	Friends For Sale!	5,351,271	+164,341	+3.07
19.	SPP Ranch!	265,854	+139,277	+52.39
20.	Country Life	8,156,867	+138,984	+1.70

As we’ve noted before, RockYou has encouraged the growth of Zoo World in part by actually putting a copy of the game within Birthday Cards; players who get hooked will probably tend to go ahead and install the regular version of Zoo World for daily play.

However, the app company doesn’t seem to be hanging onto its players very well. Between a fifth a quarter of the average successful game’s users will visit daily; Zoo World, by comparison, is at 13 percent.

To think of it another way, Zynga, the most successful gaming company on Facebook, has about 70 million daily users. RockYou has roughly a third of Zynga’s MAU, but it would have to multiply its DAU by more than 10 to reach those numbers:

Skipping down to Gangster City, this new Mafia Wars-style game from Playfish is continuing to do fairly well, although it isn’t exploding. Little Warrior, from an unknown developer, popped up in the rankings a couple weeks back with GC, but is growing even more unevenly.

Two more to point out: My City Life and My Town. As the names indicate, both of these are city builder / Sims-style games. Broken Bulb Studio’s My Town has been appearing consistently on our lists of top growers, but it’s worth noting that My City Life, though it appeared more recently and is from another unknown developer, is currently growing more quickly.