Measurement
CTR
Click-through rate
The share of impressions that produced a click.
CTR = clicks / impressions
What makes it wrong.
The mechanism, not the warning.
The denominator is not the same everywhere. Some platforms report a rate against served impressions and some against viewable impressions, and the second is always the higher number for the same creative.
In detail.
CTR is the cleanest available signal of whether a creative is working, because both of its terms come from the same system and the same moment. It is the metric to watch when diagnosing fatigue, and the one to distrust when comparing across placements.
A feed placement and a story placement produce different rates for the same creative, so a blended CTR across placements mostly measures the placement mix.