According to VentureBeat, Zynga's activities on Facebook accounted for $445 million (12 percent) of the social network's revenue in 2011. This information comes from Facebook’s initial public offering filing with the Securities and Exchange Commission today. What the numbers show is that social gaming, and Zynga's brand of it in particular, is an integral part of Facebook's ecosystem.
Zynga has more than 235 million monthly active users on Facebook who play FarmVille and CityVille for free, but often purchase virtual goods and in-game extras with real money. Around 30 percent of that revenue goes to Facebook because they charge a transaction fee for the use of the its virtual currency. Zynga also generates revenue for Facebook by spending money on ads designed to lure in new players.
Of course, the relationship goes both ways and if Zynga decided to start its own social network or simply do business elsewhere it would probably hurt the company.
Facebook filed an IPO of $5 billion, which is expected to generate $100 billion for the company.