They said it could never be done. They were wrong.

As reported this morning by Inside Social Games, CityVille has surpassed FarmVille as the number one game on Facebook. In just 20 days, CityVille has reached 60,000 monthly average users (MAUs), surpassing FarmVille’s current 56,000 MAUs. In daily average users (DAUs), which is not as impressively big a number but is more relevant to the business, CityVille has hit 16,778,004 DAUs, compared to FarmVille’s 13,357,004 (Appdata).

So, how did this happen? We already posted our analysis of why CityVille is one of the fastest growing game in the history of video games.

Here’s the summary again: CityVille was launched in 6 languages so it has spread globally. Though Facebook has removed game notifications from the main feed, it is starting to make the game notification feed more prominent on its web site which is making a difference. For all the less game spam that is on Facebook, Zynga has made up for it by sending notifications to anyone who gives their email address (which is almost impossible not to opt-in for in a Zynga game). CityVille is a good game that has been worked on for over a year before launching (most Facebook games are launched after a few months when they should be incubated for many months more). And finally, Zynga is cross-promoting CityVille like crazy within its other games, including the aforementioned FarmVille.

Which brings up an interesting point. How much of CityVille’s can be attributed to cannibalizing the traffic of its other games, such as FarmVille and FrontierVille? CityVille’s feat is based on FarmVille’s current 50,000,000 MAU statistic, not its height of 80,000,000 MAUs of last year.

Someone with more time and an aptitude with numbers than I should research this. Based on my initial analysis, there is cannibalization going on, but not significant enough to account for CityVille’s huge growth. My guess is that 5% and 10% tops of the game’s growth could be attributed to Zynga’s current players switching to CityVille.

Which means, most of CityVille’s growth can be attributed to natural growth and in the words of The Carpenters, “we’ve only just begun.” Now that CityVille has surpassed FarmVille, the question is how high will it go?

Speaking of Farmville, Inside Social Games also reported that a Chinese-version of FarmVille has been launched. Just like when Zynga launched FarmVille on the iPhone, it’s not a given that FarmVille will take China by storm. There is tons of competition for social farming games in China. But, it’s a huge market and after a few days, the game has grown to 300,000 players. That’s a good start.

This, of course, may be a dress rehearsal for when a Chinese version of CityVille in launched in the Middle Kingdom.