In 2015, Google, Facebook, Baidu, Yahoo, and Microsoft accounted for 19% of all the global ad spend flowing through all media, according to media agency ZenithOptimedia's 2016 "Top Thirty Global Media Owners" report.
The report ranks the world's biggest media companies in order of revenue generated in financial year 2013, the latest year for which it has consistent revenue figures for all the publicly-listed companies. Media revenue includes advertising but also things like circulation revenues for newspapers and magazines.
All the familiar names are in there as well as some huge foreign entities you may not recognize.