While game development on social networks is increasingly becoming more sophisticated, leading social game developer Zynga is already figuring out how to drive significant revenues out of its game network.

Long rumored in the app developer community as the developer of one of the “million dollar a month” apps in its Texas HoldEm poker game (mostly from people buying virtual currency), Norwest Venture Partners principal Tim Chang told paidContent today that Zynga is making “$30 to $40 million per year.” In addition, Nicholas Carlson at SAI reported today that his sources say Zynga turned in 2008 revenues of $50 million.

Game developers are paying close attention to the growth of the virtual goods markets within social platforms. Zynga is a somewhat unique case given its size (said to be over 150), but there are still high school students out there who will be able to pay their own way through college with the money they’re making through Facebook and MySpace games…