Microsoft Corp. is in talks about a possible investment in Facebook Inc. that could value the social networking site at $10 billion or more, according to reports.

The Wall Street Journal reported that Redmond, Wash.-based Microsoft has joined Mountain View, Calif.-based Google Inc. in expressing interest in Facebook.

Microsoft could take up to a 5 percent stake in the Palo Alto-based company, the Journal reported, which could mean about $300 million to $500 million.

The Journal also reported that Microsoft and Facebook could expand an agreement -- set to expire in 2011 -- in which Microsoft serves display ads to Facebook.

Facebook has raised about $40.7 million so far, and earlier this month announced a new $10 million fund to promote development of new applications for its online social network site.