Sending Affiliate Conversions to Facebook CAPI via Postback

If you run Meta ads to an affiliate offer, the Facebook pixel will under-report by 30–60%. The fix is Meta's Conversions API (CAPI), fed by an S2S postback from your affiliate network.

The data Meta needs

• event_name: usually Purchase or Lead.
• event_time: unix seconds when the conversion happened.
• action_source: website.
• event_id: dedupe key — reuse your order ID.
• user_data.fbc: built from the original fbclid.
• custom_data.value and custom_data.currency.

Building the fbc value

The fbc string follows a fixed format:

fb.1.<event_time_ms>.<fbclid>

You don't need cookies — once you receive the fbclid back through your postback's SubID, just construct fbc on the fly.

The CAPI request

POST https://graph.facebook.com/v18.0/<PIXEL_ID>/events
?access_token=<TOKEN>

{
  "data": [{
    "event_name": "Purchase",
    "event_time": 1718200000,
    "action_source": "website",
    "event_id": "order_12345",
    "user_data": {
      "fbc": "fb.1.1718200000000.AbCdEfG..."
    },
    "custom_data": {
      "value": 49.00,
      "currency": "USD"
    }
  }]
}

Deduplication with the pixel

If your landing page also fires a Purchase pixel, set the same event_id on both. Meta will keep one and drop the other — no double-counting.

End-to-end flow

1. Capture fbclid on landing, send to network as SubID.
2. Network fires postback with the SubID on confirmed sale.
3. Your endpoint builds fbc + payload, POSTs to CAPI.
4. Meta credits the original ad and uses the signal for bidding.

Verifying it works

Use Events Manager → Test Events. Add test_event_code to your CAPI payload. You should see events arrive in near real time with Match Quality at least Great.