Twitter made an estimated $45m in ad revenue in 2010

"Twitter could more than triple its ad revenue this year but will still trail far behind Facebook, a new report claims.
Market research firm eMarketer predicted today that Twitter will generate about $150 million in advertising revenue this year, up from the $45 million it made last year. The vast majority of this year's revenue--$140 million--will come from inside the U.S., the firm said. eMarketer expects Twitter ad revenue to reach $250 million next year."

Source:  eMarketer estimates, reported by Cnet, 24th January 2011