FrontierVille, for instance, launched a few months earlier, initially took off, zooming past 30 million users.

FORBES: FarmVille No Longer Facebook's Top Application

Like FarmVille, FrontierVille is about virtual crop-tending, but the settings and game-play are more developed (a sort of FarmVille 2.0).

FORBES: Zynga's FrontierVille Reaches 20 Million Virtual Pioneers

Its new FrontierVille game saw initial success, rocketing to 20 million users in its first five weeks after launching last June.

FORBES: FarmVille Players Down 25% Since Peak, Now Below 60 Million

Facebook credits are virtual currency that can be spent on virtual goods (virtual farmland, outfits for your avatar, virtual food, treasure, new abilities, etc.) within apps and social games played on Facebook, like FrontierVille and Farmville.

FORBES