Only in the relaxed world of social networking could a deal valuing a company at $15 billion (£7.3 billion) be revealed so casually.

Eschewing formal announcements, Facebook informed the world that Microsoft would buy a 1.6 per cent stake when its head of public relations “befriended” Microsoft’s marketing chief on her profile page. The move, which came after a series of rumours about an impending deal between the two groups, was seized on by web gossips at Valleywag, a technology site, and the game was up on one of the biggest ever bets on the internet.

Yesterday Microsoft described its deal with Facebook, under which it will invest $240 million in the social networking site, as being a great win.

The implied value for the entire company is $15 billion, even though it has failed to break even and has predicted revenues of only $150 million this year. Analysts and other industry observers questioned whether the gamble would pay off.

David Bradshaw, an analyst for Ovum, said: “Facebook is not worth that implied multiple at the moment. Whether it will be in the future is an open question.”

Microsoft’s aims are clear: the move marks the latest stage of a grand plan to transform itself into a web advertising powerhouse, giving Google a run for its money. As the web’s hottest property, with an audience of close to 50 million active users, Facebook, it believes, could play a central role in helping it to achieve that aim.

The deal — for which the Seattle giant batted aside competition from Google — is not only for a stake, but for the right to become the exclusive third-party advertising partner for Facebook and to sell its advertising internationally. Kevin Johnson, president of Microsoft’s platform and services division, said: “Expanding this partnerships will position Microsoft . . . to better take advantage of advertising opportunities around the world.”

Suspicions have been raised that Microsoft will get access to Facebook’s often highly personal user data. The two groups yesterday avoided questions on that point, saying only that “user trust is core to what we focus on”.

The critical question, say analysts, is whether Facebook’s huge popularity can be maintained. “People are valuing the site on assumptions of growth that are very uncertain,” Mr Bradshaw said.

However, Steve Ballmer, Microsoft’s chief executive, recently highlighted the fickle nature of social networking sites such as Facebook. “I think these things are going to have some legs, and yet there’s a . . . faddish nature about anything that appeals to younger people,” he said.

The decline of sites such as Geocities, acquired by Yahoo! in 1999 for $3 billion, show how potential goldmines can fail to live up to hopes. Nevertheless, Yahoo! has become no less reluctant to get involved in the phenomenon, declaring when asked about a possible tie-up with Facebook: “Where you have large numbers of people engaging with each other online Yahoo! sees great opportunities for monetisation.”

A survey this year by Parks Associates fuelled questions about the surging valuations of such sites, revealing the “chronic unfaithfulness” of users. Yesterday, for example, some users threatened to leave if the relationship of the site and its big corporate partner, Microsoft, got any closer.

Other analysts said that Microsoft was relying on Facebook increasingly becoming a “home page” for web users — the route by which they contact their friends and enter the net.

Toan Tran, an analyst at Morningstar, said: “The only way this works is if Facebook becomes a sort of users’ operating system on the internet.”

Mr Johnson said Facebook could attract as many as 300 million users worldwide within the next few years.

For Facebook, the deal, and its decision to hold out for a fresh offer despite an alleged offer last year from Yahoo! valuing the whole company at $1 billion, looks like a clear winner.

Facebook’s founder, Mark Zuckerberg, becomes a paper billionaire. His group will use its funding to hire more staff and develop its site further, finding new ways to keep users enraptured.

A 1.6 per cent stake, analysts said, was also not enough to prevent Google from taking ownership eventually. The Microsoft deal could instead make the search giant all the more determined to get a piece of the action.

Face: the facts

Facebook

Number of users more than 49 million active users and 55,000 regional, work-related, collegiate and high school networks

Hits 54 billion page views per month; more than half of active users return daily

Users’ average stay in August 14 minutes

Growth 3 per cent weekly growth since January; doubling of registered users since September 2006, when Facebook opened up to users outside colleges and universities

Sixth busiest site in the US

MySpace

Number of users more than 200 million - 69 million unique visitors in August

Hits 67 billion page views in August

Users’ average stay in August 26 minutes

Growth approximately 230,000 users added per day

Bebo

Number of users more than 34 million

Hits 7 billion monthly page views

Microsoft last night unveiled a rise of 23 per cent in profits, aided by sales of the Halo 3 video game and buoyant demand for personal computers carrying its Windows operating system. The group said net profits for the three months to the end of September had jumped to $4.29 billion, from $3.48 billion a year ago.

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