In a conference call with journalists and analysts, Kevin Johnson, president of the platforms and services division at Microsoft, described the deal as a “major advertising syndication win for Microsoft.”

“The equity stake that we are taking in Facebook is a strong statement of confidence in this partnership,” Mr. Johnson said. “It’s a statement of confidence in the fact that our advertising platform is going to get stronger and will help monetize Facebook.”

Image Dustin Moskovitz, an early Facebook employee and a vice president, spoke yesterday at a cellphone industry conference. Credit... Kimberly White/Getty Images

On the same call, Mr. Van Natta responded to a question about reports that both Yahoo and Google were bidding to invest in Facebook by saying, “We were very fortunate to have a lot of folks interested in a partnership with us around advertising.”

Mr. Van Natta said the investment would allow the company to more than double its work force to 700 employees by the end of next year. He also said it would help Facebook expand internationally and buy the technology to help keep up with its rapid growth. Facebook says it has 50 million active members and is adding 200,000 new ones each day.

MySpace, the site owned by the News Corporation that is Facebook’s main rival in social networking, has more than twice as many members but is growing more slowly.

The two companies left several questions unanswered. Facebook hinted that it had other investors alongside Microsoft, but declined to say who they were or how much money they had injected into the company. It was also unclear whether Facebook would involve Microsoft as it continued to develop its own advertising network to serve ads on portions of its site. Facebook has plans to introduce an advertising system at an event in New York City next month.