[It looks like not all of the bad offers had been taken out of FishVille's offer wall, we are disappointed to say. In the latest embarrassment to the industry, TechCrunch's Michael Arrington took a closer look at some of the offers in the FishVille offer wall provided by DoubleDing, and noted some apparently scammy ones. He also discovered that his account was blocked from seeing these offers, while others weren't.

Here's what Zynga told him in response: We asked all offer provider networks to remove the mobile category. Upon learning today that one provider was still showing 6 ads, we asked them to remove these too. They told us they hadn’t realized this was still in their testing queue and immediately removed them.”]

Zynga, the largest developer on Facebook — and one that has come under increasing scrutiny recently for its use of scammy advertising offers in games — has just soft-launched a new title called FishVille.

The company trademarked the name “FishVille” recently, so the launch is perhaps no surprise. But the move was very quiet, as the app currently has 61 users — and no, we didn’t forget a few zeros in that number.

The game is what you get when you cross the very popular new genre of Facebook apps built around the virtual aquarium idea, together with Zynga’s ruthless ability to take popular genres and create its own hit to compete against entrenched leaders.

The company has done the same with Mafia Wars among role-playing games, with FarmVille among virtual farming apps, and most recently Café World among virtual restaurant apps. Those titles are now at or near the top of our leaderboard of the largest apps overall on Facebook’s platform.

FishVille, in particular, is taking aim at recent winners like Happy Aquarium from CrowdStar. That game has recently surged past a number of big Zynga titles to reach 24.0 million monthly active users, and it is now the fifth largest app on the entire Facebook platform, according to AppData. Others, like Fish World, have also gotten millions of users. In fact, we’re not quite sure how many big fish games there are on Facebook — there are a few significant ones made by Asia-based developers, for example. Virtual aquarium games, from our understanding, first got big in China, before they got big as apps on Facebook.

Fishville itself is pretty straightforward. You start with a virtual aquarium, with a few fish in it. You harvest the fish, sell them, use your earnings to buy fish eggs, then feed the fish. If you’ve played FarmVille, you’re probably familiar with this dynamic. Other features include the ability to invite friends, and earn additional coins and experience by visiting their tanks. As with other games, Zynga has users wait certain amounts of time as their fish grow — a good method of keeping people coming back for more.

And, of course, if you need some extra coins in order to buy more fish, you can purchase them through the usual variety of direct payment methods, or earn them through offers. A look at Zynga’s offer wall shows that things are pretty clean now — yes, there are mobile quiz ads, for example, but these are actually allowed by Facebook if the offers do not mislead users about what they’re getting into.

Growth to come?

Zynga, no doubt, plans to cross-promote this app with its other popular titles, as well as buy ads on Facebook to help get it in front of more users. Those two strategies have been key to how it has grown other games.

But Zynga has also excelled at optimizing “viral channels” to reach new users,  Before Facebook changed its news feed from a raw stream to a slower, algorithmically-tuned one a couple weeks ago, Zynga had mastered the art of getting users to share information from games into the stream. This means users were seeing massive amounts of stories about FarmVille, etc. — and now they’re not, so much. Zynga’s games seem to have done fine since the redesign rolled out on October 23rd, from the little data we have available to work with (see more, here and here).

	Name	MAU
1.	FarmVille	63,304,313
2.	Causes	35,209,300
3.	Café World	28,477,669
4.	Mafia Wars	25,782,246
5.	Happy Aquarium	24,002,808
6.	Pet Society	21,468,688
7.	FamilyLink.com (formerly We’re Related)	20,522,599
8.	YoVille	19,735,219
9.	Texas HoldEm Poker	18,757,583
10.	Farm Town	18,380,105

So we’re very interested to see if Zynga can pull off a repeat of Café World, which launched at the beginning of October and since then has grown to be the third-largest app, with 28.5 million monthly actives as of today.

The wild card — or actually, a lot of wild cards — will be the changes that Facebook is planning to introduce to its platform this month. Features like notifications will shortly be folded in to users’ inboxes. For example, the app currently uses the one-line feed item story to automatically share user actions back to their profile walls — a practice that users sometimes find surprising, and one that won’t be available for too much longer.