CTRs aren't irrelevant, since they're a concrete proxy for conversions that ad networks can relay back to publishers quickly, but nobody actually pays by clicks anymore. That was common fifteen years ago, but CTRs plummeted circa 2002 and never recovered. Display ad revenue is now invariably calculated from impressions.
I work in Facebook app monetization and mobile ads, it is almost all CPC and CPI. The ad ecosystem is diverse enough that different pricing models apply in different situations.
