Twitter has over 600 advertisers

"[Twitter CEO Dick Costello] said that over 80% of Twitter advertisers come back and renew and claimed that average engagement rate with ads on Twitter was of “orders of magnitude” better than traditional media advertising.
He gave an example of a Volkswagen ran ad for new VW Beetle and said the promoted tweet had engagement rate of 52%.
The claim comes in stark contrast to recent reports that suggested ads onTwitter do not work very well for some advertisers. The report said that advertisers were having mixed results.
He also cited a Radio Shack campaign called ‘You Need a New Phone’, which while it only ran for one day on Twitter it resulted in store exchanges being up by double-digits for the next three days.
“These are amazing statistics that marketers just can’t believe when they first hear them…So the business is working phenomenally well,” Costolo said.
However, Costolo would not be drawn on whether Twitter was profitable. He snapped shut saying only “Not gonna talk about it”.
Twitter had been projecting to have around 100 advertisers by the end of last year. It ended up with 150 and now has 600-650 advertisers.
“We’re in no hurry to go make sure [that we jam Twitter up with ads]. It just doesn’t make sense.”"

Source:  BrandRepublic's Wallblog, 2nd June 2011