What changed
Third-party cookies and device identifiers degraded, and with them the inferred audiences that most targeting relied on. Buyers stopped paying for guesses about who someone might be and started paying for known identity.
Which quietly repriced something you already own: the names, emails, phone numbers, and purchase history sitting in your invoicing system.
Three things to do with it
Retarget your own customers. Existing customers are cheaper to reach and far likelier to buy again. A list upload targeted at people who bought two years ago and have not been back is the highest-return audience most local businesses have.
Exclude. Just as valuable and almost never done. Stop paying to advertise a new-customer offer to people who are already customers, or to reach the person who bought yesterday.
Build lookalikes. Platforms can find people resembling your best customers — but only if you tell them who those are. Upload the top decile by value, not the whole list.
How to get it clean enough
Export name, email, phone, ZIP, last purchase date, and lifetime value if you have it. Deduplicate. That is sufficient — platforms hash and match on their side, and imperfect lists still match at usable rates.
The barrier is almost never data quality. It is that nobody ever made it a task.
Keep ownership
Whoever runs your media should hold access to your accounts and audiences, not title to them. If a provider builds audiences that live in their own account, you are renting your own customer list back. Run this ad account ownership checklist before you sign.
Get it in writing before the first upload, not after the relationship sours.