Zynga has been very quiet the last few months, after a string of big launches last fall. Its only game since PetVille in early December is Poker Blitz, which launched earlier this week but is not yet being promoted heavily (judging by its traffic numbers).

But now Zynga appears to be focusing the big trend of this past winter: SimCity-style city building games on Facebook.

The company has been running ads directing users to answer a few questions on social gaming, then offers them $75 in exchange for taking a 30 minute survey. Inside Social Games contributor Eric von Coelln spotted the ads yesterday, and noted that Zynga asks two question specifically about Playdom’s genre-leading Social City. The game has surged to 6.15 million monthly active users and 2.51 million daily active users in just a few weeks, according to AppData, our independent traffic measurement service.

The first is “How frequently do you currently play Social City” and the second is — quite specifically — “Into which of the following ranges does your current Social City level fall?”

Zynga is clearly trying to figure out just how much users are playing the game. These questions should tell the company a lot about the success of the game. If people play the game all the time and have reached higher levels, then Zynga will know it has a serious competitor. If most people say they barely play and are at low levels, Zynga can better discern if most of the traffic is coming from ads or other non-organic sources.

Either way, the company’s interest in city-building suggests it either has a title in the works already, or is about to start one. We haven’t seen any city-building trademarks come through for Zynga — although it has many others — so the name of the app is anyone’s guess. CityVille? TownVille? SimVille? Take your guesses in the comments.