Everyone likes to talk about how dependent Zynga is on Facebook, but that relationship cuts both ways. In the social network’s S-1 filing for its $5 billion IPO, Facebook says that Zynga accounted for 12 percent of its revenue in 2011, through a combination of virtual goods payments and advertising.

“If the use of Zynga games on our Platform declines, if Zynga launches games on or migrates games to competing platforms, or if we fail to maintain good relations with Zynga, we may lose Zynga as a significant Platform developer and our financial results may be adversely affected,” Facebook says.

Despite the fact that they clearly need each other (or maybe because of it), Zynga and Facebook have had rocky relationships in the past. Zynga has been trying to reduce its dependence by  pursuing with things like partnerships with other companies and by developing its own gaming portal. Still, it’s probably safe to assume that it will be pretty Facebook-reliant for the foreseeable future.