It turns out, Twitter can make money.

That’s according to a new report from eMarketer, which is “cautiously optimistic” about ad revenue prospects for the micro-blogging service now that it’s ramping up services for marketers. The research firm expects Twitter’s revenue to hit $150 million in 2011 — more than three times last year’s $45 million estimate.

What’s more, ad revenue could soar to $250 million in 2012 if the site can grow its user base and persuade advertisers that it is a “go-to secondary player” to social-networking giant Facebook. Facebook will likely grab $4 billion in ad dollars this year alone, the report added.

Twitter, hampered by fear that aggressive advertising through its platform could turn off users, has been slow to monetize its service. Meanwhile, advertisers are looking for ways to tap the vast social network.

The company — run by CEO Dick Costolo — counts 175 million online users who send short notes, in 140 characters or less, to friends and followers, which is a far cry from Facebook’s user base of 600 million.