On Wednesday, as Google preannounced the initiative, OpenSocial seemed like a coalition of the weak, boasting minor social-networking players such as Plaxo, Flixster, and Google's own Orkut. With the addition of MySpace, however, OpenSocial becomes a substantial challenge to Facebook.

"OpenSocial is going to be become the de facto standard for developers right out of the gate," Chris DeWolfe said at the press conference. "It will have access to 200 million users, making it way bigger than any other platform out there."

Google chief executive Eric Schmidt indicated Facebook can participate in OpenSocial. "The most important principle about openness is that everyone is invited to join," he said.

A Shift in Power

With MySpace's partnership, Google has transformed the social-networking landscape, several observers said. "This is very important because it could shift the social-networking power eventually to this OpenSocial consortium and allow this group to redefine what social networking will be in the future," Tim Bajarin, principal analyst with Creative Strategies, said in an e-mail.

Michael Arrington, publisher of the TechCrunch blog, said "Google has pulled off an absolute coup." The "coup" leaves Facebook -- which recently took $240 million from Microsoft for a 1.6 percent share -- to decide whether it will join or turn up its nose at openness.

In a statement, a Facebook spokesperson said: "Facebook has still not been briefed on OpenSocial. When we have had a chance to understand the technology , then Facebook will evaluate participation relative to the benefits to its 50 million users and 100,000 platform developers."

Several anonymous sources told various press outlets that Facebook had, in fact, been talking with Google. And Facebook developers were spotted Thursday night at a campfire gathering for top developers held on Google's campus. "We're not talking to the press," they were reported as saying.

Facebook Likely To Go It Alone

Bajarin said he would be "surprised" to see Facebook support OpenSocial since "they have momentum today and a development environment of their own that is gaining traction and, if executed well, could help them keep their leadership position." OpenSocial is likely to become the major competitor to Facebook over time, he added.

But TechCrunch's Arrington said he believes Facebook eventually will join the OpenSocial force "because suddenly Facebook is being painted as the closed outsider while everyone else is allowing the use of nonproprietary coding platforms and portability of applications," he wrote.

While that may be true, Facebook is not a walled garden in the same way the old AOL was, Bajarin said, because "it includes an actual applications development environment and Web 2.0 approaches that could dramatically enhance their property."

Google will release an Orkut "sandbox" shortly with which developers can try out the new OpenSocial programming interfaces, executives said at the campfire event. Joe Kraus, Google's director of product management, said developers were stabilizing things. "We want reasonable assurance over the next week or two that there are not any other major breaking changes. We want to make it open to the public as soon as possible," he said.