Microsoft is making sure that Facebook runs Microsoft ads with a hefty investment into the social network company.

Microsoft buys 1.6% for $240 million and defines the long guessed valuation of Facebook therefore with $15 billion.

“We are pleased to take our Microsoft partnership to the next level,” said Owen Van Natta, Chief Revenue Officer, Facebook. “We think this expanded relationship will allow Facebook to continue to innovate and grow as a technology leader and major player in social computing, as well as bring relevant advertising to nearly 50 million active users of Facebook.”
Facebook continues to experience strong growth both in the U.S. and international markets. Almost 60 percent of Facebook’s users are outside the U.S. With an average of 200,000 new users registering each day, Facebook continues to be one of the most-trafficked sites on the Internet.

On Aug. 22, 2006, the companies announced a U.S. only strategic alliance that named Microsoft the exclusive provider of standard banner advertising on Facebook using Microsoft’s digital advertising solutions and the Microsoft adCenter platform. In early 2007, the terms were extended to 2011.
Via this Microsoft press-release.