This year, driven by display ads, mobile will account for 69.9% of all digital advertising. With a 33.9% share of total US ad spend, mobile will pass TV as the leading advertising medium in the world’s largest ad market—and we expect that share will grow to a whopping 47.9% by 2022.

"Advertisers are pouring dollars into mobile due to growing mobile commerce activity. Conversions from mobile display ad placements have already surpassed those of desktop," said Corey McNair, forecasting analyst and author of eMarketer's latest report, "US Ad Spending: Facebook and Google to Capture Over One-Quarter of the Market."

Meanwhile, redesigned mobile shopping sites are improving the surfacing of product information and reviews, leading to higher conversions from search ads.

Overall, mobile advertising will grow more than three times faster than total media this year, 23.5% vs. 6.6%. Transitioning competition from traditional to mobile formats is leading to escalating ad prices as brands vie for mobile placements. Advertisers’ increasing spend will push mobile’s share of total media to 47.9% by 2022.

In the latest episode of "Behind the Numbers," we further discuss the ad spending landscape.