An example
Acme Watches runs a retargeting ad set to 40,000 recent site visitors. In week one, frequency is 1.8 and CTR is 1.6%. By week four, frequency is 5.2, CTR is 0.9% and CPA has gone from $38 to $61. The ads did not change. The same small audience simply saw them too often.
Why it matters
Small audiences, like retargeting pools or narrow interests, wear out fastest because there are few new people left to reach. Fatigue costs money quietly: Meta keeps spending, but each extra impression lands on someone who has already decided. A common starting point is to look closer when frequency passes 3 and CTR has fallen about 20% from its early level, but that is a rule of thumb, and the right line varies by account and audience size.
How to improve it
Widen the audience or merge small ad sets so there are more new people to reach. Rotate in fresh creative so repeat viewers see something new. For retargeting, shorten the window or exclude recent buyers so the pool keeps refreshing.