Google and Facebook controlled 64% of the US digital ad market in 2015

"Google is reportedly jumping deeper into the ad blocking issue by exploring an acceptable ads policy, publisher and industry sources say.
Google, together with Facebook, controlled 64 percent of the [US] digital ad market last year. With that clout, Google could have an outsized impact on the kinds of ad formats that become the industry standard."

Source:  Digiday, 9th May 2016

(Note - this includes some ad formats where they pass on a large percent of the revenue to others)