Facebook's late push into mobile advertising is paying off big time. Less than two years after the company began serving targeted ads to mobile users, the social network is by far the second largest player in the quickly growing market. eMarketer expects Facebook
to rake in $6.8 billion from mobile ad sales this year, trailing only Google
which remains the clear industry leader.eMarketer
's data shows how heavily concentrated the mobile ad market is. In the past year, the Top 4 ad publishers accounted for more than 70 percent of mobile ad revenues and, perhaps even more impressively, Google and Facebook alone pocketed two thirds of every dollar spent on mobile advertising in 2013.