According to the New York Times Microsoft is going to Pay $240 Million for Stake in Facebook:

"…The two companies said on Wednesday that Microsoft would invest $240 million for a 1.6 percent stake in Facebook. The investment values the three-year-old Facebook, which will bring in about $150 million in revenue this year, at $15 billion.

The deal ends two months of jockeying between three major Internet players for the right to invest in and forge close ties with Facebook.

As part of the deal, Microsoft will sell the banner ads appearing on Facebook outside of the United States, splitting the revenue with it. Last year, Microsoft struck a deal with Facebook to run banner ads on the site in the United States through 2011.

The astronomical valuation for Facebook is evidence that Microsoft executives believed they could not afford to lose out on the deal."

I suppose this kind of thing is inevitable as Microsoft does not want to fall any further behind Google than it already is.