SAN FRANCISCO (Bloomberg) - Facebook Inc., the second most-popular social-networking website, hired Google Inc. veteran Sheryl Sandberg as operating chief, mining its rival's ranks to gain expertise in online advertising sales.

Ms Sandberg, 38, will start on March 24, Palo Alto, California-based Facebook said today in a statement. She joined Google, the most popular search engine, in 2001 and served as vice-president for global online sales of the company's ad products.

Facebook is bolstering its management team to cope with a fourfold surge in users in December from a year earlier and rising demand from advertisers. The company is battling Google and social networking rival MySpace for sales in a market that may almost triple to $2.5 billion in the US by 2011, according to research firm EMarketer Inc. in New York.

Ms Sandberg will help Facebook build its operation globally, managing sales, marketing, and business development, Facebook said. She will report to Mark Zuckerberg, the company's 23-year-old CEO.

David Fischer, vice-president of online sales and operations, will take over Ms Sandberg's duties, Google spokesman Matt Furman said in an e-mail.

Google fell $18.14 to $438.88 at 2pm New York time in Nasdaq Stock Market trading. The stock had declined 34 percent this year before yesterday.

Ms Sandberg, who has an MBA from the Harvard Business School, oversaw ads sold through Google's website. She helped co- founders Larry Page and Sergey Brin start Google.org, the company's philanthropic arm.

Prior to joining Google, Ms Sandberg was chief of staff at the US Treasury under President Bill Clinton. Before that, she was a consultant with McKinsey & Co.

The number of visitors to Facebook surged more than fourfold to 97.8 million in December from a year earlier, according to researcher ComScore Inc.

The site gained on MySpace, owned by Rupert Murdoch's News Corp., which had 107.2 million users, an increase of 19 percent, ComScore said.

In July, Facebook hired Gideon Yu, the former chief financial officer of Google's YouTube. Google paid $1.65 billion for YouTube in 2006.

Microsoft Corp., the world's largest software maker, purchased a 1.6 percent stake in Facebook for $240 million in October, valuing the company at $15 billion.