The 3 companies in the last few years have been able to build billion dollar advertising revenue streams. A combinations of factors like – Audience data, own devices/ecosystems, improved measurement, mobile apps & others – allowing advertisers to reach new audience streams & those who are in the more advanced stage of the custoemer journey.
At a broad level Ads can be categoried to across
1) Search – Google, Bing & others
2) Publishers – Media, newspapers, programmatic & others
3) Social Media – FB, twitter, etc
4) New Ad giants – Amazon, Walmart & Apple