Twitter Inc. founders Biz Stone and Evan Williams (from left) are pictured at their San Francisco office. Twitter messages are limited to 140 keystrokes. Revenue has been missing so far, but the firm expects to start making money this year. (Associated Press)

Twitter Inc. founders Biz Stone and Evan Williams (from left) are pictured at their San Francisco office. Twitter messages are limited to 140 keystrokes. Revenue has been missing so far, but the firm expects to start making money this year. (Associated Press)