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Mark Zuckerberg, founder and chief executive officer of Facebook Inc., speaks at the Computer History Museum in Mountain View, California, U.S., on Wednesday, July 21, 2010. Facebook Inc., the world's most popular social-networking service, reached 500 million users today, up from about 400 million in February, Zuckerberg said. Photographer: David Paul Morris/Bloomberg via Getty Images

Mark Zuckerberg, founder and chief executive officer of Facebook Inc., speaks at the Computer History Museum in Mountain View, California, U.S., on Wednesday, July 21, 2010. Facebook Inc., the world's most popular social-networking service, reached 500 million users today, up from about 400 million in February, Zuckerberg said. Photographer: David Paul Morris/Bloomberg via Getty Images