EMarketer sees 2014 mobile ad spending at $31.5B

Worldwide spending on mobile advertising is expected to reach $31.5 billion this year, a 75 percent increase from 2013 thanks largely to Facebook and Google, according to a new report by research firm eMarketer.

By the end of this year, eMarketer expects mobile to account for nearly a quarter of total digital ad spending, which is estimated to reach $137.5 billion. That's up from 15 percent of the $119.8 billion digital ad spending total in 2013.

Mobile ad spending has been growing fast in the last couple of years, mainly due to ad revenue growth at Facebook Inc., the world's largest social network, and Internet search giant Google Inc.

In 2013, spending on mobile advertisements more than doubled, to nearly $18 billion from $8.8 billion in 2012. Facebook and Google accounted for most of that increase. Together, their mobile ad revenue rose $6.9 billion.

EMarketer expects Facebook's share of the mobile ad market to grow to 21.7 percent this year from 17.5 percent in 2013, taking a bite out of Google's share. Google remains No. 1, with half of the mobile advertising market in 2013. This year, that's expected to decline to 46.8 percent.