As much as I agree with the core of what you are saying, it isn't AdMob that is the issue. It's that Google controls both the marketplace, the pub-side and the buy-side, as well as the browser and the devices used to experience ads. Apple is now aiming for a similar level of control in the ad space very blatantly using their ownership of the platform to do what they want to power themselves up.

This vertical integration in the space is what drives the anti-competitive behavior, if the Google buy-side were a separate company that happened to acquire AdMob there would be no real issues as they don't control the whole stack.

FB on the other hand sells a service and lets companies use their APIs to integrate as well as competing with them via their internal sales team.

These companies can do this only because you are forced to work with them because they own such a large part of internet traffic, not because they offer particularly good services or have better technology or prices.