Web advertising dollars are transitioning from desktop to mobile much more quickly than previously anticipated -- and that has eMarketer boosting its 2013 mobile-advertising forecast. According to the firm, U.S. mobile advertising will be a $7.29 billion industry in 2013, $100 million more than eMarketer projected in December 2012.

Clark Fredricksen, eMarketer's VP-communications, said the revision was made because mobile advertising is growing faster than originally expected. Facebook's and Twitter's "explosive entrances" into mobile advertising and a Google's continued "strong performance" on mobile are the driving factors behind the growth, Mr. Fredricksen said in a statement.

In 2011, Facebook and Twitter earned no revenue from mobile. This year, they are expected to earn $964.9 million and $266 million on mobile, respectively.

Google -- the recipient of more than half of all mobile ad revenue -- is projected to increase its mobile display revenue in coming years as it improves mobile monetization of YouTube, eMarketer said.

While eMarketer was bullish on social and search-based mobile advertising, it was comparatively pessimistic about Millennial Media. eMarketer revised down its forecast for Millennial Media's share of U.S. net mobile-internet display ad revenue, saying that the company's traffic acquisition costs are no longer expected to fall in the next two years. Millennial's share of U.S. mobile display revenue is expected to decrease from 2.8% in 2013 to 2.6% in 2014.

iAd is projected to fare better. eMarketer predicted Millennials's net mobile ad revenue to increase by 47.7% from 2013 to 2014 while iAd's will jump by 76.6%.