MOUNTAIN VIEW, Calif. — Internet social networking leader MySpace is joining Google Inc.'s platform for sharing applications across the Web — a concept that threatens to undermine the rapid growth of their common rival, Facebook Inc.

Google trumpeted the MySpace coup Thursday in a meeting with reporters, two days after revealing its plans to create a distribution network for interactive applications known as widgets.

The programs — created by a hodgepodge of independent software developers and other Web sites — make it easier to share music, pictures, video and other personal interests on social networking sites.

MySpace, owned by News Corp., was conspicuously absent from the initial list of Web sites that agreed to host the widgets from Google's OpenSocial platform.

That raised questions whether MySpace might try to build its own proprietary platform, much like Facebook has done.

But MySpace and Google executives said they began discussing an open-ended system that culminated in OpenSocial more than a year ago. The formal announcement about the alliance was timed to coincide with a party that Google is throwing for software developers Thursday evening.

Google also disclosed for the first time that another popular social networking site, Bebo

.com, will host widgets supplied from its platform, which is trying to create a common coding standard for the applications so they work on hundreds of Web sites.

Other previously disclosed participants networks include social networks Friendster, hi5, LinkedIn, Ning and the Google-owned Orkut.

All told, OpenSocial's potential audience is expected to exceed 200 million people.

But OpenSocial was an unimpressive alternative to Facebook's platform until MySpace confirmed its participation, Gartner analyst Ray Valdes said.

"This is more likely to get developers' attention," Valdes said Thursday.

Although Facebook has been growing faster, MySpace remains the Internet's biggest social network — a hangout where people look for dates, share their passions, make new friends or just connect with familiar faces.

In September, MySpace's U.S. audience totaled 68 million, compared with 30.6 million for Facebook, according to the latest data from comScore Media Metrix.

Google's one-size-fits-all approach contrasts with Palo Alto-based Facebook's, which relies on unique coding that has prevented widgets developed for its sites from working at other places on the Web.

Facebook's formula has been highly effective, spawning more than 8,000 widgets in the five months since the platform started. Including visitors from outside the United States, Facebook says it now has 50 million members and has doubled in size since May.

Facebook's booming membership encouraged Microsoft Corp. to pay $240 million for a 1.6 percent stake in Facebook last week, a deal that valued the three-year-old startup at $15 billion.

Now, it looks as though Google and MySpace are forming a tag team to duel Facebook and Microsoft. "This clarifies the battle lines, but it's not just a two-way conflict," Valdes said.

That's because other large Web sites like longtime Google rival Yahoo Inc., online auctioneer eBay Inc. and Internet retailer Amazon.com Inc. haven't picked a side yet. It's also possible that those Web sites might introduce competing platforms for social networking widgets.