A good way to contextualize how concentrated the digital advertising market has become:
Methodology:
Estimates are based on the analysis of various elements related to the ad spending market, including macro-level economic conditions; historical trends of the advertising market; historical trends of each medium in relation to other media; reported revenues from major ad publishers; estimates from other research firms; data from benchmark sources; consumer media consumption trends; consumer device usage trends; and eMarketer interviews with executives at ad agencies, brands, media publishers and other industry leaders.