Mark Zuckerberg, founder and CEO of Facebook, delivers a keynote address at the company's annual conference in San Francisco, California July 23, 2008. Zuckerberg, 24, told an audience of 1,000 industry executives, software makers, media, and his mother and father at the conference here of plans to let Facebook features run on affiliated sites off the social network. REUTERS/Kimberly White (UNITED STATES) NEW YORK, Oct 27 (Reuters) – Social networking sites like Facebook and LinkedIn raise “serious new challenges” for financial regulators, the head of the largest U.S. independent securities regulator said on Tuesday.