It took CityVille less than a month to become the most popular game on Facebook

"Today, CityVille added an astounding 6.9 users to end the day at 61.7 million monthly active users, compared with 56.8 million for FarmVille, according to stat collector AppData. Before today, FarmVille, also made by Zynga, was the biggest social game on Facebook, and it also happened to be the biggest game on any platform in terms of the sheer volume of users.
Now the challenge will be to make money from all of those users. If Zynga hangs on to them, it stands a chance of becoming a far bigger player in social games than it already is.
When CityVille took off shortly after its launch on Dec. 2, it was clear that its growth path was going to break all records. We knew this would happen, but it’s still pretty astounding. It’s going to be a pretty Merry Christmas for Zynga. Zynga’s traffic on Facebook has now soared to 261.6 million monthly active users, which is probably near its all-time peak. In November, Zynga had around 198 million monthly active users."
Source:  VentureBeat, 24th December 2010