Investors are staying optimistic about the future of online gaming, judging by funding news this week. Three more companies have announced venture rounds, including Wooga, Playfire and Six Degrees Games. We covered the biggest one yesterday: Social game developer Playdom’s $43 million first round.

The optimism is not surprising, given the revenue estimates for online gaming in general. For example, the virtual goods business model will bring in $1 billion in the US alone this year, according to our Inside Virtual Goods report. But venture investors want exits, not just revenues — and there was a watershed one this week, when Playfish announced its sale to Electronic Arts in a deal worth up to $400 million.

Berlin, Germany-based Wooga, the creator of popular Facebook quiz game Brain Buddy, announced today that it has raised €5 million in funding in a new round led by Balderton Capital, with existing investor Holtzbrinck Ventures participating. Brain Buddy launched in July, and currently has 5.65 million monthly active users and more than half a million daily active users. The company also says that it plans to launch two more games in the coming weeks, presumably on Facebook.

Playfire, a United Kingdom-based social networking service for console and PC games, raised £1.3 million ($2.1 million) earlier this week, from Atomico Ventures and some high-profile angel investors. While many traditional video games don’t contain many social features, Playfire features include a profile, in-game player statistics, a way to track favorite games, and notifications about relevant news stories, videos and site discussions.

Back here in the US, Marina Del Rey, Calif.-based Six Degrees announced a $7 million round led by Time Warner investments for its ActionAllStars.com sports game. Founded by entrepreneurs with experience in mobile gaming and sports television, the company’s game lets you create virtual athletes to compete against each other in a virtual world. The company also plans to begin selling virtual goods, like sport items or avatar customizations, according to VentureBeat.