Microsoft last night thwarted its arch rival Google to clinch a $240 million (£117 million) deal for a key 1.6 per cent stake in Facebook that values the hugely popular social networking website at a hefty $15 billion.

Microsoft saw off intense competition from Google for the strategic stake, in return for which the software titan will become the exclusive third-party seller of overseas advertising outside the United States on Facebook’s site, which has now amassed more than 49 million internet users.

The deal with Microsoft extends its existing agreement with Facebook under which it already sells US advertising for the site. Although the site, which lets members chat online and post data such as photographs, is free to users, revenues from advertisers on its pages are potentially vast.

The $15 billion valuation of Facebook implied by last night’s deal comes after the website’s investors, which include Accel Partners and Greylock Partners, held out for a price close to the top end of what analysts considered feasible.

Reacting to the agreement last night, analysts said that the $240 million paid by Microsoft looked steep and amounted to a gamble that Facebook will be able to transform itself into an entry portal and hub for a wide range of internet activity by a range of users.

“The only way that this works is if Facebook becomes sort of the users’ operating system on the internet - [that] everyone logs into Facebook every day to get in contact with friends and uses a multitude of future applications that will be developed for it,” Toan Tran, an analyst for Morningstar, said.

This month, Facebook opened the door to outside software developers for the first time, attracting 4,000 applications. The social networking site will allow these developers to profit from their applications, which will include games, widgets and other, more practical tools. The move is expected to help to increase Facebook’s value still further.

It is only 13 months since Mark Zuckerberg, Facebook’s founder and chief executive, opened the website to users from beyond its original student base.

Charlene Li, an analyst for Forrester Research, said that Microsoft’s success in reaching the deal last night reflected its status as a better strategic fit for Facebook, since the software group knew better how to work with program developers and build computing environments.

“Microsoft is a company that knows how to build platforms, knows how to develop relationships with developers,” she said. “Frankly, Google did not bring as much to the deal.” Facebook, which it is thought Mr Zuckerberg hopes to prepare for a float, competes with the larger MySpace, which currently has 100 million users.

Only last week, Rupert Murdoch, the chairman and chief executive of News Corporation, owner of MySpace and parent company of The Times, ruled out buying Facebook, saying that it was overvalued. News Corp bought MySpace for $580 million in 2005.

Facebook received 6.5 million unique visitors in August, compared with MySpace’s 6.4 million, according to Nielsen/NetRatings, the analysts. Both sites are visited by one in five Britons with online access.

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