Lookalike audiences vs uploaded buyer lists: which seed actually scales

Lookalike audiences vs uploaded buyer lists: which seed actually scales

The setup: An e-com offer with ~1,200 past buyers on file. Build a lookalike off the buyer list, or off the pixel's broader 'add-to-cart' event? $1,500 test.

The move: Two 1% lookalikes — one seeded from 1,200 actual purchasers, one from ~15,000 add-to-cart events. Same creative.

The numbers (illustrative): Add-to-cart lookalike (big seed): scaled smoothly, +14% ROI, lots of volume. Buyer-list lookalike (small but pure seed): +33% ROI but capped — the audience was small and saturated within a week, then ROI collapsed as frequency climbed.

The lesson: Seed quality vs seed size is a real tradeoff. A pure buyer seed gives a sharper, higher-converting audience that exhausts fast. A bigger, noisier intent seed scales further at lower margin. Neither is 'right' — it depends whether you need margin or volume this week.

What I'd do differently: I ran the pure buyer lookalike hard and hit the saturation wall blind, watching ROI rot from +33% to -5% over five days. Use the pure seed for a high-margin burst, switch to the broad seed before frequency kills it. Watch frequency on small audiences like a hawk — it's the saturation alarm.