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FILE - In this Sept. 22, 2011 file photo, Facebook CEO Mark Zuckerberg smiles during the f/8 conference in San Francisco. As Facebook promotes the vision of its 28-year-old CEO as part of this week's first-ever sale of stock to the public, one of the most striking features of his persona is the contradiction between the public and private that remains at its center. Zuckerberg avoids questions about himself and once sued a magazine for publishing documents revealing details from his past. (AP Photo/Paul Sakuma, File)