Facebook ads are really popular, ~84B in revenue. I believe Microsoft still operates a large ad network. Taboola is everywhere. Amazon has a huge ad network

Facebook ads are really popular, ~84B in revenue. I believe Microsoft still operates a large ad network. Taboola is everywhere. Amazon has a huge ad network as well, ~14B in revenue, though obviously limited in terms of what is advertised. Verizon Media also apparently operates a sizable ad network.