Adding one field to our CAPI payload moved match rate from 66% to 79% overnight

The useful part

Adding hashed phone number to an already-live CAPI integration — nothing else — pushed match rate 13 points in a single day. That's the kind of result that makes you realize the integration being "on" and the integration actually working are two different things. That feels complete because email is the obvious identity field, but Meta's matching logic doesn't weight all parameters equally across all users.

How it works

• In Korean markets especially, a large share of accounts are tied to Kakao-linked emails — addresses users created for one service and never check.
• Below 70% match rate, the delivery system starts filling gaps with probabilistic inference instead of resolved user signals.
• Good creative, solid audience, reasonable budget — and the campaign quietly underperforms because the signal layer has a leak nobody went looking for.
• Meta appears to use it as a tiebreaker when other signals conflict.
• I expected external_id to be redundant given the email was already matching.

What to take from it

What I've tracked across the accounts I run: sustained match rate in the 65-69% range correlates with a 25-40% increase in effective CPM within 7-10 days. Accounts that drop under 60% often stall in learning phase entirely, even when they're clearing 50+ weekly events that should qualify them to exit. The full parameter set that actually moves the number: em, ph, fn, ln, ct, st, zp, country</co...

Details worth keeping

Most of them are only passing hashed email (em). The phone number is the stronger identity anchor.