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.

The publishers of this article owns a rival ad network. The difference between PR pieces and news is getting thinner.

https://newscorp.com/2017/12/05/news-corp-launches-news-iq/

(from putlake's comment above)

Off the top of my head, googles Display Product has quite a few. - Magnite (Formerly, Rubicon) - Xander (Formerly, AppNexus) - GumGum - Index Exchange - Facebook - Sovrn - Unruly - Sharethrough - Triplelift - OpenX - Amazon A9 - Conversant - Yahoo (AKA Gemini) - Outbrain - Revcontent - Medigo

That's most of the bigger players, but there are hundreds. Many aren't worth the time, and are spun up when one rep takes some budget from a large brand and does "Direct" deals and tries to start their own shop with "Guaranteed CPMs". It's all bullshit

There's quite a lot of networks.

It's not a complex business to start, you know.