While Google does have Facebook as a real competitor, that competition is not on the ad networks side. Google controls the vast majority of ad networks sales (i.e. ads placed on publisher's sites)

Google makes a significant portion of the value for each ad.

> For displaying ads with AdSense for content, publishers receive 68% of the revenue recognized by Google in connection with the service. For AdSense for search, publishers receive 51% of the revenue recognized by Google. [ 0]

[0] https://support.google.com/adsense/answer/180195?hl=en