By the end of this year, eMarketer says mobile will account for nearly a quarter of total digital ad spending.
Mobile ad spending has been growing fast, mainly due to ad revenue growth at Facebook Inc. and Google Inc.
In 2013, mobile ad spending more than doubled, to nearly $18 billion from $8.8 billion in 2012. Facebook and Google together saw an increase of $6.9 billion in mobile ad revenue.
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, though Google remains No. 1.