SEATTLE - MICROSOFT beat out rival Google in a battle to invest in socialising website Facebook, agreeing to pay US$240 million (S$351 million) for a roughly 1.6 per cent stake in the Web phenomenon and expand a deal to sell advertising.

Microsoft and Facebook said the US$240 million investment valued Facebook at US$15 billion, which analysts said was a steep price and a bet the young company would be able to transform itself into a hub for all sorts of Web activity.

'The only way this works is if Facebook becomes sort of the users' operating system on the Internet - everyone logs into Facebook every day to get in contact with their friends and use a multitude of future applications that will be developed for it,' said Morningstar analyst Toan Tran.

Facebook, a social network that lets friends and colleagues share information among themselves, already has allowed developers to create games and other applications specifically for its site, whose popularity makes it extremely valuable to advertising sellers such as Microsoft and Google.

Some 250,000 new users register on Facebook each day, Microsoft and Facebook said.

Microsoft said it would be the exclusive third-party advertising platform for Facebook, which has more than 49 million Internet users. That extends a previous deal into Facebook sites outside the United States.

Google and Microsoft, now rivals for Internet-based audiences and applications, each expressed interest in a minority stake in Facebook for its growing user base and advertising potential.

The rivals have butted heads before for Internet properties. Google beat Microsoft with a US$1.65 billion acquisition of online video sharing site YouTube last year.

Forrester Research analyst Charlene Li said that Microsoft was a better strategic fit for Facebook, since it knew how to work with software developers and build computing environments - such as its Windows operating system.

'Microsoft is a company that knows how to build platforms, knows how to develop relationships with developers. Microsoft developed the network that is the biggest, most vibrant one out there,' she said. 'Frankly, Google didn't bring as much to the deal.'

Google Co-founder Sergey Brin told a meeting with Wall Street analysts at the company's Silicon Valley headquarters that his company could partner with important websites.

'We don't feel, at a higher level, that we need to own every successful company on the Internet,' he said.