[Update: Facebook has responded to let us know that the below stats are due to a bug in their reporting, which they're currently working on fixing.]

Our Monday list of fastest-gaining Facebook apps by monthly active users is more than a bit out of the ordinary this morning. We’re seeing huge growth for several apps over the weekend, in a pattern that looks very similar between each affected app.

Regular readers may recall that the same thing happened in mid-June; it turned out that Facebook had made an internal change to show the previously unknown, but accurate stats for a number of apps like Quiz Planet! and Facebook for Android. Has the same thing happened again? We’re not sure yet, but if so it’s big news for developers.

Take a look at the AppData top 20:

	Name	MAU	Gain	Gain,%
1.	Café World	35,324,196	+15,100,354	+75%
2.	Texas HoldEm Poker	41,597,080	+13,612,033	+49%
3.	Mafia Wars Game	24,142,623	+6,429,541	+36%
4.	Zoo World	12,857,340	+6,053,805	+89%
5.	PetVille	17,823,978	+3,861,130	+28%
6.	Birthday Calendar	7,662,413	+3,761,963	+96%
7.	FrontierVille	24,984,631	+3,598,109	+17%
8.	Windows Live Messenger	6,138,142	+2,666,076	+77%
9.	Restaurant City	13,017,032	+2,289,626	+21%
10.	Phrases	23,693,492	+2,194,455	+10%
11.	Happy Island	8,681,928	+2,044,392	+31%
12.	Sorority Life	4,978,879	+1,750,713	+54%
13.	Market Street	1,828,168	+1,674,500	+1,090%
14.	Hallmark Social Calendar	2,154,008	+1,480,988	+220%
15.	Drinks for All!	2,131,663	+1,463,687	+219%
16.	Friends For Sale!	3,377,538	+1,402,706	+71%
17.	Circle of Moms	2,592,284	+1,210,558	+88%
18.	Who thinks of you?	1,176,603	+1,050,504	+833%
19.	Zoosk	6,318,780	+989,944	+19%
20.	QuizBone	3,350,837	+927,701	+38%

The top three apps on the list show a combined gain of about 35 million MAU. As it happens, they’re all Zynga games. That company shows a gain of almost 50 million MAU over the weekend.

Prior to today, it was widely-accepted fact that Zynga had lost about 50 million MAU since April. While it’s possible that Facebook is experiencing a bug in reporting — we’re still trying to get responses from both Facebook and Zynga — it also seems possible, given the latest data, that Zynga never lost ground at all. We’ll be following up on that story over at Inside Social Games.

But the gains aren’t limited to games. Windows Live Messenger, Hallmark Social Calendar, Friends For Sale!, Circle of Moms and Zoosk all show a pretty much identical kick in growth over the weekend to the games at the top of the list. Here’s a representative example:

We’ll post an update when we know more about what’s happening.