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When someone clicks a Facebook ad, Facebook appends fbclid to the URL (for example, ?fbclid=abc123). Facebook’s pixel script reads it and stores it as a first-party cookie (_fbc) with a version prefix, timestamp, and click ID value.
  • FBCLID is the URL parameter.
  • FBC is the cookie that stores it.
Upstack includes the FBC cookie value when sending events via the Conversions API (CAPI). Facebook compares it against its records — mismatches trigger diagnostics warnings.
Upstack also preserves click IDs across sessions and devices via its identity graph. A visitor who clicked a Facebook ad last week and returns today via Google can still have their original click ID restored.

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FBC expiry

Facebook click IDs are valid for 90 days. After that, Facebook considers them expired. Upstack stores click IDs beyond this window so returning visitors can still be matched, but if Facebook no longer recognizes the value, it may trigger a “Modified FBC” diagnostic.

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FBP (Facebook Browser ID)

FBC ties to a specific ad click, but Facebook also sets a second first-party cookie called _fbp. FBP identifies the browser regardless of whether the visitor came from an ad. Both are sent with every Conversions API event.

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FBC is one of Facebook’s strongest matching signals

FBC directly links a conversion back to a specific ad click, which is why it carries more weight than email or phone for attribution. A high percentage of missing or modified FBC values can reduce your Event Match Quality score in Events Manager.

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Common reasons FBC goes missing

  • Redirects (age gates, A/B tests, language selectors) can strip the fbclid parameter before the pixel reads it.
  • Ad blockers can prevent the cookie from being set in the browser — which is why Upstack also sends events server-side.

Modified FBC value warning

Generate Facebook CAPI key

Configure Meta Pixel destination

Attribution mismatch troubleshooting