Microsoft, the largest software maker, views the stake as a strategic investment reflecting the pair’s aim to combat Google Inc., rather than as a near-term moneymaker, said the person, who requested anonymity because the plans are private. Microsoft held 26.2 million Facebook shares, or 1.7 percent, after the initial public offering, regulatory filings show. At Thursday’s close, the stake was worth $551 million.

Some of Facebook’s biggest investors, including Microsoft and Goldman Sachs Group Inc., get the chance to start selling shares on Aug. 16, after a ban on sales expires. The stock has lost 45 percent since the May 17 IPO through Thursday, and the prospect of further sales has weighed on the share price.

Microsoft paid $240 million for the stake in 2007 in an investment that valued Facebook at $15 billion. The two companies are collaborating on features that integrate social information from Facebook into Microsoft’s Bing Web-search service, a rival of Google’s market-dominating product.

Tony Imperati, a spokesman for Redmond, Washington-based Microsoft declined to comment, as did Ashley Zandy, a spokeswoman for Menlo Park, California-based Facebook.

Facebook climbed 3 percent to $21.64 at 3:25 p.m. in New York, while Microsoft retreated 0.7 percent to $30.30.