Glossary
What are fbclid, _fbc and _fbp?
fbclid is the click identifier Meta appends to your landing-page URL when someone clicks a Meta ad. _fbc and _fbp are first-party cookies set by the Meta pixel: _fbc stores that click ID and _fbp identifies the browser. Both can be sent as the fbc and fbp parameters in Conversions API events to improve matching and attribution.
The formats are documented and predictable: an _fbc value typically looks like fb.1.{timestamp}.{fbclid} (the creation time plus the click ID from the URL), and _fbp like fb.1.{timestamp}.{random number}. In Conversions API events they travel inside user_data as fbc and fbp.
fbc is one of the strongest signals for attributing a conversion to the ad click. The problem is survival: Safari’s tracking prevention caps script-set cookies at seven days (less in some cases), private browsing and cookie purges remove them, and a blocked pixel never sets them at all.
That is why robust tracking setups capture the fbclid at landing and store it first-party, server-side. If the _fbc cookie is missing at conversion time, a valid fbc parameter can be reconstructed from the stored fbclid and timestamp. The click then still gets credit weeks later, wherever the visitor is recognized.