Yahoo has joined Google and MySpace to form the three musketeers of social media. The companies announced Tuesday they are starting the OpenSocial Foundation to create universally-accepted standards for social networking sites and applications.

Yahoo’s endorsement of Google’s OpenSocial initiative comes two weeks after MySpace (NWS) opened its doors to developers using the OpenSocial standard. MySpace was the first social networking site to adopt OpenSocial.

Yahoo (YHOO) did not disclose which of its web properties will use OpenSocial. “We’re supporting OpenSocial because it’s rapidly growing and maturing,” said Wade Chambers, Yahoo’s vice president of platforms.

The OpenSocial Foundation plans to provide a formal intellectual property and governance framework for developers. Google OpenSocial director Joe Kraus argues that as the web becomes more social and social networking sites open up their platforms, developers will benefit from using a set of common standards. Kraus says the nonprofit will set up shop in 90 days.

“Open source is important,” Kraus told Fortune last week. “It gives [sites] and developers the confidence that they can use OpenSocial in perpetuity without concern of something bad happening. It’s always available to them and it’s not going to be obsolete.”

Google (GOOG) announced its OpenSocial initiative last fall one week after Microsoft (MSFT) agreed to pay Facebook $240 million for a minority stake. Facebook became the first social network to open its platform to developers last May. Facebook, which uses its own open-source platform, says it does not have any immediate plans to join the OpenSocial Foundation.