That number comes from both the 30% transaction fee Facebook takes for all Facebook Credit purchases, and also from the direct purchase of advertising on the site by Zynga itself. According to the filing, this number is up from both 2010 and 2009, where Zynga accounted for 10% and 9% of Facebook's total revenue, respectively.
Where Facebook is dependent on Zynga for profits, so too is Zynga dependent on Facebook for its established user base and therefore possibilities for viral growth. However, with the continued development of Zynga Direct (the company's attempt at an "instant social" platform outside of Facebook), we could finally see the Big Z pull farther away from the social network, even to the point of freedom. Will we ever get to that point, where we can play Zynga's games without Facebook? Only time will tell.
Are you surprised that Zynga accounted for this much of Facebook's total revenue, or would you expect the number to actually be higher? Sound off in the comments.