SAN FRANCISCO -- Research firm EMarketer Inc. projects that Facebook's mobile advertising revenue in the United States will quintuple next year. That would put Facebook just behind Google in the mobile ad business, EMarketer analyst Debra Williamson said. But Facebook, which has been slow to embrace mobile and is now trying to reinvent itself as a "mobile first" company, has a long way to go, Williamson said. Next year, Facebook will generate $387 million in U.S. revenue from ads on mobile devices, EMarketer projects, or about 9% of overall U.S. mobile ad sales. EMarketer estimates that Facebook will generate about $73 million in U.S. revenue from ads on mobile devices this year, about 4% of the $2.6-billion U.S. market for mobile ads. EMarketer expects Facebook to grab nearly 10% of that market by 2014. Currently Google has about 55%, EMarketer estimates. And mobile ad revenue will account for about 20% of Facebook's overall U.S. ad sales by 2014, Williamson said. "For Facebook to be a mobile first company would require it to have more than 3% of its total revenues come from mobile, which is the number for this year," Williamson said. Even two years from now, it's 20% of their total U.S. ad revenues, which isn't huge when you think about it. She characterized the growth of Facebook's mobile ad business as "steady progression." "Facebook is not going to transform into a mobile advertising powerhouse overnight," Williamson said. Mobile ads are one of the top concerns for investors who worry that Facebook had not kept up with more than half of its 955 million users who access the service on phones or other mobile devices. Facebook's stock price has suffered as doubt about its ability to generate mobile ad sales has intensified. A Facebook spokeswoman said in an emailed statement: "We are a mobile-first company. That reality informs everything we do and all the products we create, both for people and brands. We are focused on scaling advertising products that take full advantage of the News Feed, which is where our mobile users spend the majority of their time. While it's still early, the results that marketers are seeing on our mobile platform are positive. In a recent interview, Gokul Rajaram, Facebook's product director of ads, said the entire company from Facebook Chief Executive Mark Zuckerberg on down has committed to "mobile first." When you take a product to Zuck to review, the expected first thing you are going to talk about is the mobile version of the product. That applies to ads, too," he said. This is the first time that EMarketer has forecast mobile ad revenue for Facebook, which has become increasingly crucial to Facebook and other companies hoping to follow their users as they shift from the desktop to mobile devices. Bottom line, Williamson said: The desktop is still where Facebook makes its money. The bulk of their ad revenue still comes from ads on the right-hand side of the page on the desktop. None of that is funneled toward mobile at this point," she said. And Facebook hasn't yet figured out how best to show ads to mobile users, Williamson said. It first started showing mobile ads to users in June. "A lot is going to have to happen this year to get mobile advertising to perform well for the users and the advertisers," Williamson said. I can see that Facebook is doing a lot of tweaking and testing. Until they hit on a successful format for users and advertisers, I think the growth is going to be relatively slow. Is Mark Zuckerberg in over his hoodie as Facebook CEO? Facebook CEO Mark Zuckerberg won't sell shares for one year Facebook stock rallies after Mark Zuckerberg says he won't sell Follow me on Twitter @jguynn