Twitter has 2,400 advertisers

"Twitter makes money three ways, says Bain. Twitter sells promoted tweets, promoted trends, and ads. Many brands, for example, want to know how to increase their followers. "We've scaled from next to no advertisers to more than 2,400,' says Bain.  "We have about 100 folks working directly with advertisers and agencies all over.  We thought were going to have to answer the question, 'Why Twitter?' But no advertiser has that question.""

Source:  Twitter Chief Revenue Officer Adam Bain, quoted in SFGate, 30th November 2011

Earlier - 1,600 advertisers announced in early October