zyngaZynga has become the hottest social gaming company around, with more than 196 million users on Facebook alone. Today, the San Francisco company raised $15.18 million in an extension to its second round of funding, according to a regulatory filing.

No new investors were listed in the filing, according to VentureWire. The company previous raised about $40 million in venture capital from Kleiner Perkins, Foundry Group, Avalon Ventures, Institutional Venture Partners and Union Square Ventures. That’s just the latest piece of big news for Zynga, whose FarmVille game is the No. 1 app on Facebook, with 65 million monthly active users, according to AppData. NPR’s Weekend Edition even had me on over the weekend to discuss the FarmVille phenomenon, since the game has become the largest online game in sheer numbers of active users.

The company was also named one of the hottest new brands. In an interview with AdAge, chief executive Mark Pincus said Zynga’s next frontier is smartphone games. The company has expanded onto the iPhone but has met with limited success there. Meanwhile, Zynga is still trying to figure out how to handle offers right, since a scandal broke out that showed Zynga games had scam-like offers attached to them as alternative payment systems. Zynga makes most of its money through virtual goods in its free games. With that business model, gamers play games for free but purchase virtual goods such as tractors that help them tend their crops better in FarmVille.