• by Gavin O'Malley  @mp_gavin, September 8, 2015

While the researcher expects Google to remain the leader in mobile through 2017, Facebook’s share is growing at a higher rate, and the gap is expected to narrow slightly over the next several years.

Facebook will narrow the divide in overall digital spending, growing its market share from 13.2% in 2015 to 16.1% in 2017, eMarketer expects.

“As it continues to leverage its unique targeting capabilities, we expect Facebook to increase its share of ad budgets from brands, direct response advertisers and small businesses," eMarketer analyst Martín Utreras explains in a new report.

By contrast, Google's share of the total digital ad market in the U.S. will shrink from 40.1% to 35.4% during the same period.
This year, mobile should account for 77% of Facebook's digital ad revenue, and grow to 85% by 2017.

Along with the obvious boost from more video, eMarketer expects ad revenues generated by Instagram to drive Facebook’s ad growth.

"In the coming quarters, we expect Instagram and video ads monetization to keep increasing Facebook's share of the display ad market," according to Utreras.

This year, Instagram is expected to earn $570 million in ad revenues, or more than 10% of Facebook's total mobile ad revenues, according to eMarketer. By 2017, Instagram's ad revenues will grow over four times in size to reach $2.39 billion -- or 28% of the company's total mobile ad revenues.

More broadly, Facebook is expected to generate $7.66 billion in display ad revenue in 2015 -- growing nearly 44.9% over last year.

Google properties should take in $4.11 billion in display advertising this year -- 19% higher than in 2014. Display only accounts for 17.6% of its total ad revenues for the search giant.