A custom audience is a targeting segment you define from your own data: website visitors, app users, customer lists, or engagement (video viewers, lead forms). Platforms match hashed identifiers to users. Quality beats size — a 5k purchaser seed often outperforms a 500k all-visitors list for prospecting lookalikes.
Refresh lists after major sales; stale seeds degrade lookalike quality.
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Last verified Jun 5, 2026