Who would have thought that a simple city simulation would become the biggest-ever app on Facebook, eclipsing FarmVille Zynga&'s CityVille game crossed 84.2 million monthly active users over the weekend, beating Zynga&'s FarmVille, which peaked at 83.76 million users in March, 2010, according to market analyst AppData.

It&'s a sign that social interaction &8212' something that CityVille has on steroids, by not only tapping your friends, but encouraging you to help friends build out their own cities too &8212' is still seeing new forms of innovation online. True, CityVille is merely an evolution from Frontierville, another Zynga game which has similar features. But CityVille had a second force behind it: With Facebook&'s reach still growing quickly, namely in Asia, CityVille&'s growth comes on the back of an larger Facebook &8212' which arguably helps grow games even faster. The open questions are: What sort of social features will game makers devise next to enthrall users in new games this year And when does the Facebook &''growth effect&'' begin to plateau

CityVille debuted on Dec. 2 and has been the fastest-growing app and social game on Facebook. In fact, it is the fastest-growing game in history, in terms of the numbers of users signing up to play it. The game is available for free and users can choose to pay real money for virtual goods such as more energy.

Over the past week, the game has averaged gains of over 2 million monthly active users per day, according to Inside Social Games. So its total usage will likely keep breaking records. But not everybody is coming back on a daily basis. CityVille&'s daily active user count is about 16 million, compared to FarmVille&'s peak of 34.5 million daily active users. Overall, Zynga has 281.9 million monthly active users, which is either near or above its peak in the spring. The next-closest rival on Facebook is Takeoff Monkey, with just 57 million monthly active users.

CityVille passed FarmVille&'s current monthly active user count of 58 million on Dec. 24.

In the game, you can build a town with homes, shops, farms and community buildings. Though the interactivity is limited in some ways, CityVille does a good job of creating the illusion of real-time play, where lots of things are happening on the screen at the same time. As you can tell from the screen shot, your friends can help you manage the city as well. It thus appears to be more social and interactive than a lot of other Facebook games, where you generally play by yourself, taking one turn at a time.

In an interview with VentureBeat, CityVille&'s Mark Skaggs said the CityVille team was formed from scratch in the spring and that 95 percent of the team had never worked on a game before. A few were veterans like Skaggs, who previously worked at Electronic Arts. The vision was to create a game that actually felt like you were controlling a city in real time.

As I&'ve played the game, it&'s easy to see how Zynga hopes to generate revenue from it. Every move you make in the game takes energy, and your stores need inventory to sell. Both are in short supply. If you want to accelerate your progress and generate more inventory and energy, you can purchase it with real money. That helps generate revenue, since a small percentage of the players will go ahead and spend money.

Or you can visit your friends&' cities to earn more resources. You can also generate more virtual revenue for your city by opening retail franchises in other cities. These factors make the game more social. Zynga has also figured out ways to keep the engagement, or time spent per game session, higher. You can, for instance, plant some strawberries on a farm and reap the resources within five minutes. Normally, in games such as FarmVille, it takes hours or days before the crops can be harvested.

But Zynga had better be on alert as it scales this game even bigger. It has performance problems and I don&'t know why. The game has to be frequently refreshed after it stalls. And whenever you do something that requires the use of Facebook resources, such as inviting a friend to the game, it takes a long, long time to load the screen. That will turn off impatient users.

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Tags: CityVille, Farmville

Companies: Facebook, Zynga

Tags: CityVille, Farmville

Companies: Facebook, Zynga

Dean is lead writer for GamesBeat at VentureBeat. He covers video games, security, chips and a variety of other subjects. Dean previously worked at the San Jose Mercury News, the Wall Street Journal, the Red Herring, the Los Angeles Times, the Orange County Register and the Dallas Times Herald. He is the author of two books, Opening the Xbox and the Xbox 360 Uncloaked. Follow him on Twitter at @deantak, and follow VentureBeat on Twitter at @venturebeat.

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