NEW YORK (Fortune) -- Now it's Google's turn to be hounded by an upstart. Facebook, the popular social networking site, has lured top Google executive Sheryl Sandberg to serve as its chief operating officer.

Sandberg, who ran Google's online sales unit, is the first senior executive to jump from Google's management team to another company. George Reyes, Google's chief financial officer, announced his retirement in August, but is staying on until a successor is named.

News of Sandberg's exit comes amid a steady drop in Google shares. Since hitting a high of $747 in December, Google shares have dropped 41% as the days of hyper growth appear to have ended for the Net colossus.

Sandberg's move to the No. 2 job at Facebook was first reported by Kara Swisher on her BoomTown blog, which is run by The Wall Street Journal, and confirmed by Fortune's GoWest blogger Adam Lashinsky.

At Google, Sandberg ran the automated advertising operations for the search giant, the unit responsible for a large part of the company's profits and revenue.

Sandberg is expected to apply her deep operations skills to Facebook and transform the social networking site into a full-fledged business. She replaces Owen Van Natta, who announced his exit last month.

"The focus for Facebook is scaling," Sandberg said in an interview Tuesday. "That's what I've done at Google. Another similar challenge is building an advertising network, which I also did at Google."

Facebook has become the leading challenger to MySpace the online social networking unit of News Corp (NWS, Fortune 500). Facebook was founded by Mark Zuckerberg, who is also chief executive.

This isn't the first time Facebook has raided Google. In the last seven months, it brought in Gideon Yu, the former chief financial officer of YouTube, a Google (GOOG, Fortune 500) unit, as its CFO. Facebook also nabbed Benjamin Ling, the engineer credited with creating Google Checkout, to run its software platform.

Sandberg joined Google in 2001, three years before it went public.

Senior writer Adam Lashinsky and writer-reporter Yi-Wyn Yen contributed to this report.