Frequency and audience saturation
Definition
Frequency is how many times, on average, the same person has seen your ad in a period: impressions divided by reach. Saturation is what happens once the audience those ads can reach has been used up.
How it works
A campaign draws on a finite pool of people the platform judges reachable for it. Early on, most impressions land on someone new. As the pool empties, the system serves the same ads to the same people again, because that is all it has left. Later exposures persuade less and cost no less, so the cost per result rises while the campaign itself stands still. Two things refill the pool: creative the system treats as new, and a wider audience.
What to watch for
When the cost per result climbs at a flat budget, open the change history for the same weeks; if nobody touched the campaign, saturation is the first suspect. Do the arithmetic in your own account: impressions divided by reach, this month against the month the cost was last acceptable, and check whether reach has stopped growing while impressions continue. An account split into many narrow audiences saturates pool by pool, and adding one more narrow segment of the same people refills nothing. A frequency figure averaged across the whole account blends fresh campaigns with exhausted ones and hides both; read it per campaign, per period.
The question you ask
“What is this campaign’s frequency now, and what was it when the cost per result was last where we wanted it?”