Sarah McBride and Alexei Oreskovic:

Costolo said the company counts roughly 600 advertisers, up from 150 advertisers at the end of 2010.

and:

The company is expected to bring in about $150 million in ad revenue this year […]

Do take note that in December they raised 200 million dollars in VC money, yet they are set to make only 150 million of that back this year. Add to that the fact that they had 450 less advertisers last year and it makes this last statement even more comical:

Costolo declined to answer a question about whether Twitter was profitable.