, the fast-emerging group that wants to make it easier for users to share their personal contacts between different Web 2.0 and social media services, has landed perhaps its biggest fish yet: Microsoft.

Microsoft will officially announce its intention to join the international workgroup within a day or two, according to a source close to the matter.

Microsoft began talking to soon after Google, Facebook and Plaxo all announced their support several weeks ago, said the source.

The working group counts 60 members, most representing Web 2.0 firms such as Yahoo, LinkedIn, Twitter and SixApart.

Microsoft would seem to be the odd man out, not being part of the tight-knit Silicon Valley fraternity as well as a laggard in Web 2.0.

However, Microsoft is a stealth social networking giant, with more than 400 million users with accounts at Hotmail, Windows Live Messenger or both, according to an interview late last week with Adam Sohn, a director in Microsoft's online services business.

And in social media, Microsoft is a major player. According to Nielsen Online, visit statistics from last August, 2 Windows Live Spaces and MSN Groups together make Microsoft the second-largest social networking provider, behind MySpace but ahead of Facebook and Classmates Online as well as Google and Yahoo's efforts.

"Most major applications will probably be touched by social features in the coming years - and no one has a broader distribution of applications and data than Microsoft," said the source.

Microsoft has also been early in other efforts to make social media services play better together.

For instance, it pledged last February to back the OpenID Web authentication standard, a move Yahoo! did not make until last week.

OpenID promises to let users sign on once and be authenticated to log in to multiple websites.

And last May, Microsoft loosened its group on its Windows Live application programming interfaces (APIs), including its Windows Live Contacts API, through which outside services, when properly authenticated, can connect and securely download contact data from Hotmail or Windows Live Messenger users.

Google and Yahoo offer similar authorised APIs.