Analytics, Reporting, Ad management and Biding platform, Fraud detection, and way more.

This basic CRUD app is just user-facing thing, and now remember that they are making money not because of you, but because of publishers. This is where main functionality is.

There'd be more than the consumer-facing systems. Whether they require 4000 people to run it all is another question, but without a deeper look into what's running, it would be hard to surmise.

This is fitting here: http://danluu.com/sounds-easy/