With Over 20 Million Users on Facebook, CityVille Is Already a Metropolis

[Editor’s note: Zynga launched CityVille on Facebook at the beginning of the month, an ambitious move to help the company grow beyond FarmVille and other older hits. We’ll examine what’s making it a hit in the article below, but the larger point it seems to show is this: after a slow year for many of the larger developers, massive and fast growth is still possible on Facebook’s platform.]