Advantage+ Shopping Campaigns remain the highest-throughput buying format on Meta, and the least inspected. Across 41 accounts we audited between January and May — $2.1M to $86M in annualized spend — the same failure mode appeared in all but three.
The pattern
ASC's existing-customer budget cap defaults to a definition of "customer" that is only as good as the audience you feed it. In 29 accounts, the customer list synced to Meta was stale by 60+ days. The result: ASC happily re-acquired recent buyers at full prospecting CPA, and reported them as new-customer wins. Median measured waste: 11.4% of spend. Worst case: 23%.
The fix
Nothing exotic. A daily (not weekly, not monthly) sync of purchasers into the account's customer definition, a 180-day purchase exclusion for categories with long repurchase cycles, and — this is the step everyone skips — a monthly holdout that measures what ASC's "new customer" number actually contains. In 38 accounts, blended CAC dropped between 6% and 14% within four weeks, with no loss of volume.
The uncomfortable takeaway: the algorithm was never the problem. The data plumbing was. Which is the thesis of this publication, applied at the campaign level — the advertiser who controls their own data wins auctions the others don't even know they're losing.