Audience overlap analysis
When your own ad sets target the same people, you bid against yourself and pay a premium to do it. The Audience Overlap tool compares 2 to 5 audiences and shows the percentage and raw count of users they share, which is how you catch that cannibalization before it quietly inflates your CPMs. If your costs crept up for no obvious reason, start here.
How to use it
Open Audiences in Ads Manager, select 2 to 5 saved or custom audiences, and click 'Show Audience Overlap.' The tool returns a percentage of users shared between each pair plus a Venn-style visual.
Each audience must have at least 1,000 users for the comparison to run. Re-baseline against any audience in the set to see asymmetric overlap percentages.
Thresholds to act on
Above 20 to 25 percent overlap is the action threshold. Above 30 percent is a serious warning. Above 60 percent means merge the audiences.
Meta's auction has an overlap filter that prevents the same user from competing in multiple of your own ad sets in a single auction. The systemic effect is still rising CPMs because Meta picks one and discards others, shrinking effective reach per ad set.
Three remediation patterns
Consolidate overlapping ad sets into one with the combined budget.
Add the lower-funnel audience as an exclusion on the higher-funnel ad set. A 'viewed product' set should exclude 'added to cart'; 'added to cart' should exclude 'purchased.'
Split the audiences by mutually-exclusive criteria, for example interest A only AND NOT interest B.
Common overlap causes
Multiple lookalikes from similar source seeds.
Broad interest-based ad sets that share lots of users.
Forgetting to exclude purchasers from retargeting ad sets.
Running 1 percent, 2 percent, and 5 percent lookalikes from the same source in parallel, where the smaller percentages are nested inside the larger ones by design.
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