NEW YORK — U.S. mobile ad spending is expected to reach nearly $9.6 billion in 2013 and account for 22.5% of all digital ad investments, according to new figures from eMarketer.

The channel’s incredible growth — considering mobile represented just 11.9% of digital ad spending in 2012 and less than 3% of digital budgets in 2010 — is due to consumers using their mobile phones vs. desktop computers to access the Internet, causing a significant redistribution of revenues for some of the world’s largest ad platforms.

Facebook and Google are both major drivers and recipients of this growing market, domestically and internationally.

Based on eMarketer’s latest forecast of U.S. ad spending, the rapid growth of mobile ad revenues at Facebook has helped make the social network the second-largest digital ad seller in the United States, behind only Google. This year, Facebook will take in 7.4% of net U.S. digital ad dollars, or $3.17 billion, while Google will account for nearly four in 10, or $17 billion.

eMarketer had previously forecast Facebook would remain slightly behind Yahoo! this year. But the strength of Facebook’s mobile business has pushed the social network past Yahoo! Its share is now expected to decline to 5.8% in 2013, from 6.8% last year.

Microsoft’s share of U.S. digital ad revenues will fall from 6.2% last year to 5.9% this year. Microsoft’s $2.53 billion in ad revenues, a total that includes advertising on its Xbox and Skype services, is expected to grow to $2.87 billion by 2015. That’s not fast enough, however, for the tech company to hold or increase market share. Twitter will take in a comparatively small $420 million in U.S. ad revenues (a market share of about 1%) this year, eMarketer estimates.

On a worldwide basis, Google and Facebook are also the top two ad publishers, accounting for 31.91% and 5.64% of the market this year, respectively.

eMarketer has updated its forecast for digital ad revenues at major ad selling companies based on several factors, including: Twitter’s public filing of its S1, which revealed solid performance in the first three quarters of the year with significant mobile revenues; new data about YouTube and its contribution to Google’s revenues; and Facebook’s continued stellar mobile performance, with the potential to open up new inventory via Instagram as well as the announcement to include video ads in the newsfeed.

Just like on the desktop, Google and Facebook grabbed the greatest shares of net U.S. mobile ad revenues, with Facebook jumping from 9% to 16% between 2012 and 2013.

Globally, Google dominates the mobile ad landscape, with a 48.76% market share, eMarketer estimates. Facebook has seen its share of global mobile revenues explode this past year, growing from 5.34% in 2012 to 16.91% in 2013.

Mobile Internet ad spending worldwide will grow 105.9% to more than $18.15 billion in 2013, eMarketer estimates, up from around $8.82 billion last year. The advertising channel now accounts for 15.2% of all digital ad dollars spent globally, up from just 8.5% last year.