• by Mark Walsh  @markfwal, September 6, 2012

Facebook isn't ramping up its mobile ad revenue fast enough to keep up with Twitter. A new eMarketer forecast projects the microblogging service will take in $129 million in U.S. mobile ad revenue this year compared to $72 million for Facebook.

Both social media powerhouses will trail companies with more established mobile ad platforms. Google, for instance, is expected to reach $1.4 billion in mobile ad sales, fueled by the expansion of its huge search advertising business to mobile devices. Online radio service Pandora is projected to post $226 million in mobile ad revenue, with mobile ad network Millennial Media at $84 million, and Apple (iAd) at $75 million.

Overall, U.S. mobile advertising is expected to increase nearly 80% to $2.6 billion this year, and to hit $12 billion by 2016.

When it comes to Twitter, eMarketer concludes the company's rollout of new ad products, especially Promoted Tweets, has made the shift to mobile display advertising relatively simple. On most days, Twitter generates more money from mobile than on the desktop.

For Facebook, the transition hasn't been as smooth because eMarketer estimates the company makes most of its money from Marketplace ads, which don't appear on its mobile platform. Instead, Facebook has focused on extending Sponsored Stories to the news feeds of mobile users. That move will help the company generate $387 million in mobile ad dollars next year, topping Twitter's estimated $272.6 million.

Google, which claims more than half (54.1%) of the U.S. mobile ad market, is expected to continue dominating the category for the next couple of years. Excluding search, the market is more evenly divided. Pandora accounts for 20% of mobile display advertising, with Google at 18%, and Twitter, nearly 12%.

eMarketer said one key difference from its prior forecast is that the figures represent net revenues after companies pay traffic acquisition costs to partner sites. The research firm previously estimated gross U.S. mobile revenues for major ad networks.

“This change in definition has led to a reduction in share for mobile display ad networks owned by Millennial Media, Apple and Google, each of which pay back a significant portion of their gross revenues to ad publishers and partner sites,” eMarketer stated.

While mobile becomes a growing focus for publishers and advertisers, the firm projects it will represent just 1% of total U.S. ad spending in 2012. That's partly a result of smaller media buys in mobile dictated by smaller screen sizes that limit the number of impressions per page view.