UPDATE 1-Facebook ad products director to join online-payment firm Square

By Alexei Oreskovic and Gerry Shih

SAN FRANCISCO, June 27 (Reuters) - Facebook Inc's director of advertising products has been hired by Square in a significant coup for the young online-payments company.

Gokul Rajaram starts work at the San Francisco-based company next week as lead product developer, overseeing Square Register, the firm's point-of-sale product for retailers, Square said on Thursday. The product allows retailers to use an Apple iPad computer tablet as a cash register.

Rajaram will also work on other products, Square said.

Rajaram oversaw the development of products for Facebook's advertising business, which accounts for about 85 percent of the social networking company's revenue. Facebook has been under pressure to prove the effectiveness and long-term potential of its advertising services, as its stock remains far below the $38-a-share price of its May 2012 initial public offering.

With consumers increasingly accessing Facebook on smartphones, Facebook has transformed its ad business over the past year, placing greater emphasis on mobile ads that appear directly in users' newsfeeds.

Mike Hudack, who worked on Rajaram's team and was focused on measuring the effectiveness of Facebook's ads, replaces Rajaram as head of product management for Facebook Ads and Pages.

"Gokul has played a valuable part in building our advertising business. We wish him the best of luck in his future endeavors," Facebook said in a statement.

Shares of Facebook were up 2.2 percent at $24.70 in afternoon trading.

Square, one of Silicon Valley's most closely watched private companies, has made several high-profile hires in recent months as it attempts to live up to a lofty $3.25 billion valuation. It recently released a slew of new hardware and software, including a card reader stand and an e-commerce website.

Retailers are central to Square's growth plans. Customers using iPads make up nearly half the $15 billion in payments that Square says it processes on an annualized basis.

Rajaram joined Facebook in 2010, when the world's largest social networking company acquired his startup firm, Chai Labs. Preceding that, he was a product management director for ad services at Google Inc.