bizwriter writes with an article in bnet. From the article "An amended S-1 filing for Zynga's eventual IPO offers details of its relationship to Facebook. There's plenty of interesting stuff there in the open, but some partially redacted sections hint at why Zynga is so dependent on Facebook as a channel to get to its market — and why Facebook doesn't deploy its own games."
It appears that Zynga is allowed to get away with quite a bit thanks to all of the ad revenue they bring in for Facebook. Who has the upper hand in this relationship?