From the press release:

Facebook® and Microsoft Corp. today announced that Microsoft will take a $240 million equity stake in Facebook's next round of financing at a $15 billion valuation, and the companies will expand their existing advertising partnership. Under the expanded strategic alliance, Microsoft will be the exclusive third-party advertising platform partner for Facebook, and will begin to sell advertising for Facebook internationally in addition to the United States.

$15 billion valuation. Wow.

And politicians still say "social networks" are just for kids, eh?