The size of the ownership stake Microsoft will take during Facebook's next round of financing puts Facebook's valuation at a whopping $15 billion. Google had reportedly been courting Facebook as well.

In addition to the ownership piece, Microsoft will also extend its existing agreement to provide banner ads to Facebook in the U.S. With this deal, Microsoft will become Facebook's exclusive third-party ad platform, as well as provide Facebook ads internationally.

"This is about placing a big bet on the future of Facebook and positioning Microsoft possibly for an outright acquisition later, as well as keeping Facebook away from Google," said analyst Greg Sterling from Sterling Market Intelligence.

During a conference call after the announcement, Owen Van Natta, Facebook's vice president of operations and Chief Revenue Officer, didn't acknowledge Google was one of the company's suitors.

He said Facebook chose Microsoft because of its reputation as one of the world's top technology providers. "We were fortunate to have a lot of folks who wanted to partner with us around advertising," he said.

Kevin Johnson, president of Microsoft's Platforms and Services Division, called the deal a "big vote of confidence" for Microsoft's advertising strategy.

However, he said advertising is just one area of convergence for the companies, though he declined to mention specifically in what other areas of technology or business the two companies will collaborate.

Facebook will likely devote the cash influx from its next financing round to bankroll the torrid growth it is expecting in the coming 12 months in usage and headcount.

"This is clearly good for Facebook, as they get a big pile of cash to expand, and Microsoft has given them the valuation they were looking for," Sterling said. "So Facebook gets a big chunk of money and a massive valuation."

With about 300 employees now, Facebook expects to have about 700 a year from now, its CEO and co-founder Mark Zuckerberg said last week at the Web 2.0 Summit.

Meanwhile, the site is growing its usage at breakneck speed, with about 250,000 new users registering every day.

Founded in 2004, it currently has about 49 million active users today, up from 12 million in December. Over half of its active members return to the site daily. Some 59 percent of its users are outside the U.S.