[glesica]

I don't necessarily disagree that CTR might not be the most relevant metric here, but at some point their revenue is limited by the CTR. If they have a billion page views per day, one ad per page (for simplicity) and a CTR of 0.1%, then their daily revenue is capped at 100 million * p where p is the average price per click.

So even if users coming from Facebook are more likely to actually buy something than those coming from Google (which would make Facebook ads more valuable than Google ads, all else equal), the CTR still creates a cap on how much they can earn.