But that’s set to change next year, eMarketer predicts.

Twitter will generate  $129.7 million from mobile advertising this year, according to an estimate from research firm eMarketer. That outpaces Facebook Inc., which will generate $72.7 million..

Both Twitter and Facebook launched their first mobile ad formats this year. Twitter’s mobile ad unit is Promoted Tweets, which appear in a consumer’s timeline like any other Twitter message, which is called a tweet. Like a regular tweet, the messages will appear in the timeline once. As the shopper scrolls, the Promoted Tweet flows with the rest of the Tweets in her timeline.

Facebook rolled out three mobile ad units this year. Sponsored Stories highlight a consumer’s friends’ interactions with a brand and appear in the news feed, the first thing a consumer sees when viewing Facebook on a mobile device. The social network has also launched an ad format designed to drive consumers to download marketers’Facebook applications, including social games. And a third format lets marketers pay to show updates to their Facebook pages—such as new posts or contests—in the news feeds of consumers who have not Liked their pages.

Both Twitter and Facebook’s mobile ad revenue pale in comparison with Google, which will earn $1.42 billion this year, up from $750.0 million a year ago, according to eMarketer. Second to Google is streaming music service Pandora, which is expected to generate $226.4 million.

However, Facebook is catching up quickly. By next year, the social network’s mobile ad revenue is expected to jump 432.3% to $387.0 million and it will reach $629.4 million by 2014, eMarketer forecasts. That’s second to Google, which will generate $2.39 billion next year and $3.58 billion in 2014. Pandora is expected to be third, with $349.4 million in mobile ad revenue next year and $499.1 million by 2014, followed by Twitter, which will earn $272.6 million next year and $444.1 million in 2014.

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The overall mobile ad market is quickly rising. This year mobile ad revenue is expected to reach $2.61 billion, up 80.1% from $1.45 billion. That total will jump 69.0% to $4.41 billion next year and 50.1% to $6.62 billion in 2014. By 2016, U.S. mobile ad revenue will reach nearly $12 billion,  eMarketer says.

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