Google's ambitions seem to grow at a pace exceeded only by the soaring ascent of its stock price.

Its shares climbed above $700 today on word that the search titan is planning to take on the upstart Facebook.

Google is teaming up with smaller Facebook competitors LinkedIn, hi5, Friendster, Plaxo, and Ning to start a common platform to allow software developers to write programs for Google's social network, Orkut, as well as others. Google's initiative is called OpenSocial.

The development, reported by the New York Times, represents the clearest evidence to date that Google is serious about bolstering its social-networking offering.

"It is going to forestall Facebook's ability to get everyone writing just for Facebook," a person "with knowledge of the plans who asked to remain anonymous because he was not authorized to speak on behalf of the alliance" told the Times. OpenSocial, is "compatible across all the companies," the source said. "Facebook got the jump by announcing the Facebook platform and getting the traction they got. This is an open alternative to that."

In other words: Facebook, be afraid, be very afraid.

Google's counterstrike comes a week after Google appeared to lose out to Microsoft in the bidding to take a stake in Facebook. Microsoft has acquired a 1.6 percent stake in the social networking site for $240 million.

But industry insiders say that Google was never serious about making an investment but was merely playing along while it prepared its rival offering, which will be introduced tomorrow.

The strategy is to "out-open" Facebook. Six months ago, the site opened up its platform to third-party developers, prompting a flurry of so-called Facebook "apps," which add utility to the website. Since then, more than 6,000 Facebook applications have been launched, and at least one venture capital firm is wholly dedicated to funding them.

Joe Kraus, director of product management at Google, told the Times that the alliance's conversations preceded Microsoft's investment in Facebook. "Obviously, we would love for them to be part of it," Mr. Kraus said of Facebook. Facebook declined to comment.

Some of the hottest Facebook developers, including those behind iLike, Slide, Frixter, and RockYou, are expected to be present tomorrow night at Google's headquarters in Mountainview, California, where they will announce that they will tailor their programs to run on the OpenSocial platform.

In late morning trading today, shares of Google are up nearly 1 percent, at $700.92. In terms of market value, Google is now the fifth biggest company in the United States, surpassing Citigroup. (Google, of course, has had a better year than the bank.)