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Twitter is expected to see $259.9 million in revenue in 2012, up from $139.5 in 2011.

Over the next couple of years, the company’s revenue is forecast to reach $399.5 million in 2013 and $540 million by 2014.

Year-over-year revenue growth for 2011 to 2012 is estimated to be at 83 percent; that growth rate is expected to slow to 36 percent by 2014.

For a company that (rather famously) didn’t have a business model just three years ago, it’s been a dramatic climb.

Still these ad dollars pale in comparison to what Facebook will rake in this year, a figure in the multibillion dollar range.

It’s difficult to imagine Twitter playing at the same scale. According to previous forecasts, Twitter won’t pass $1 billion in revenue until at least 2016.

These estimates come from online intel firm eMarketer.

In an email conversation with VentureBeat, an eMarketer spokesperson said, “We forecast in early 2011 that Twitter would earn $150 in revenue that year, and that number was subsequently cut in September — a result of Twitter’s slower-than-expected rollout of several ad offerings including ad sales offices in markets outside the U.S. and a platform enabling advertisers to buy ads on a self-serve basis.”

Twitter’s money comes almost entirely from ad products, with some coming from deals to access Twitter’s Firehose API.

Late last year, Twitter introduced a new ad product that would serve promotional tweets even to users who didn’t follow the Twitter account of the brand or person in question.

Twitter CEO Dick Costolo said at a meeting three weeks ago that the ad products are selling well and that brands are happy with the results.

“We continue to see great engagement rates,” he stated. “The health of the business is great.”

One thing Twitter hasn’t done is publish a breakdown of which Promoted products are performing best or how much revenue comes from which ad products — a set of statistics we’re sure brands would love to get their hands on.

As far as valuation is concerned, other sources peg Twitter’s 2011 enterprise value around $3 billion and its market capitalization around $3.6 billion.

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