We’re seeing a few distinguishing trends continue this week on our list of the top 20 applications with the most new monthly active users on Facebook. Zynga‘s FishVille continues to surge, crossing 20 million monthly active users yesterday, riding weekly growth of 4.68 million, according to AppData.

The virtual aquarium game is now the eighth-largest app on Facebook. It is catching up with CrowdStar‘s Happy Aquarium, the formative game in the genre and still the largest with 27.3 million monthly actives.

	Name	MAU	Gain	Gain, %
1.	FishVille	20,640,953	+4,682,818	+22.69
2.	Happy Pets	5,134,179	+2,189,637	+42.65
3.	FarmVille	68,612,159	+1,770,275	+2.58
4.	21 Questions	5,162,440	+1,435,470	+27.81
5.	Café World	30,538,671	+1,082,156	+3.54
6.	Zoo World	3,178,335	+1,018,739	+32.05
7.	Snowball Fight	2,300,309	+1,017,614	+44.24
8.	Fish Isle	1,675,730	+1,005,194	+59.99
9.	Hug Me	4,466,232	+812,454	+18.19
10.	Champions Online	1,401,493	+677,693	+48.36
11.	Friend Quiz	7,270,541	+657,976	+9.05
12.	Island Paradise	7,358,436	+648,800	+8.82
13.	Ultimate Slot Machines	697,245	+626,449	+89.85
14.	Happy Thanksgiving!	665,502	+614,784	+92.38
15.	Medical Mayhem	701,038	+571,967	+81.59
16.	Give Hearts	8,603,966	+564,637	+6.56
17.	Facebook for iPhone	18,689,428	+563,461	+3.01
18.	Band of Heroes	1,269,648	+538,476	+42.41
19.	建立你的測驗	809,334	+437,103	+54.01
20.	Treasure Madness	2,004,565	+429,933	+21.45

Meanwhile, CrowdStar’s latest game, Happy Pets, has been coming up as well. It grew by 2.19 million monthly actives this past week to reach 5.13 million, making it the second-fastest growing app. Zynga’s massive FarmVille, however, was in third, with 1.77 million new users for a total of 68.6 million monthly actives.

In terms of non-game applications with growth, we have the usual quiz and gifting and poking apps. One quiz app, 21 Questions, is at number four, having grown by 1.44 million new users to reach 5.16 million; another one, further down, is Friend Quiz. Gifting and poking apps include seasonally-themed Snowball Fight and Happy Thanksgiving!, as well as RockYou‘s Hug Me and 6 waves‘s Give Hearts.

Anyway, back to games. Zynga’s Café World is on the list yet again, growing by 1.08 million to break the 30 million monthly active user mark. RockYou’s new one, virtual zoo-keeping game Zoo World, is also seeing ongoing growth, having grown by 1.02 million to reach 3.18 million in the past week.

Looking on down the list, you can also see a few other striking ones. IGG Inc.‘s Fish Isle has quickly hit the big league’s, after only launching a few weeks ago. The high-quality fish-farming game is only a month old, and an early foray by a massive multiplayer online gaming developer into social gaming. This app, and it’s developer, are off to a very solid start.

LOLapps is also back on the list again, with Champions Online and Band of Heroes both seeing significant growth.

And the new hospital genre of sim game is on, as well. We noted Medical Mayhem last week — it has you run a virtual hospital (fun, huh?) — and it has grown by 572,000 in the past week to reach 701,000. We’re assuming there will be a HospitalVille out within the next few weeks.