CityVille, the social game created by Zynga, has hit 47.9 million monthly active users in about 18 days. At this rate, CityVille may surpass FarmVille as the largest social game in a few days. FarmVille currently has 56.3 million monthly active users.
If CityVille hits this number, then it would be the fastest-growing game in history and would be the largest game ever being played today. Some of the reasons why CityVille is appealing is because there are built-in motivators to keep you playing. An example that VentureBeat gives is that when you plant strawberries in a field, you can harvest them in 5 minutes. The fastest growing crop in FarmVille takes about two hours.