There has been a lot of question about the effectiveness of Facebook advertising lately, but the numbers show advertisers are spending tons of money on it, and in mobile, it’s one of the driving forces behind a rapidly growing market.

According to a new report from eMarketer, global mobile ad spend increased by 105% in 2013, reaching $17.96 billion, and is on pace to hit $31.45 billion this year. That, the firm says, would account for about a quarter of total digital ad spend.

So which companies are leading the charge? Take a guess. Facebook and Google of course. The two companies combined for 75.2% of the additional $9.2 billion that went toward mobile, and for over two thirds of mobile ad spend in 2013, according to the report, which also says that will continue to increase this year.

“Facebook in particular is gaining significant market share,” the firm says. “In 2012, the social network accounted for just 5.4% of the global advertising market. In 2013, that share increased to 17.5%, and eMarketer predicts it will rise again this year to 21.7%. Google still owns a plurality of the mobile advertising market worldwide, taking a portion of nearly 50% in 2013, but the rapid growth of Facebook will cause the search giant’s share to drop to 46.8% in 2014, eMarketer estimates.”

While Facebook’s continues significant growth, marketers are also growing increasingly frustrated with it, not only for a decrease in organic reach, but also for the results they’re getting from ads.

This week, Forrester’s Vice President and Principal Analyst Nate Elliott blogged about how brands are becoming “disillusioned” with Facebook as an advertising platform.

“Brands and agencies are now openly talking about their discontent,” he wrote. “Every day I talk to brands that are disillusioned with Facebook and are now placing their bets on other social sites — but few of them want to go on the record. Lately, though, more brands and agencies have started speaking openly to the media about how Facebook is failing them. One former Facebook advertiser referred to Facebook as ‘one of the most lucrative grifts of all time.’”

As Jim Edwards at Business Insider, suggested in response, “The opinions of various advertisers should be taken with a pinch of salt — the numbers tell the real story,” but he added, “It is unusual for advertisers to talk this way about a major media partner. These comments will hurt.”

The numbers have been just fine for Facebook, and this eMarketer research only backs them up. In January, Facebook reported that its ad revenue was up 75% year-over-year at $2.34 billion (with 53% of that from mobile, which itself was up 23%).

Image via eMarketer