fbclid, _fbc and _fbp explained

fbclid: the click id in the URL

When someone clicks a Meta ad, Meta adds ?fbclid=... to the landing page link. It identifies that one click. It only lives in the URL, so it is gone as soon as the visitor moves to another page unless something saves it.

_fbc: the click id, saved

The _fbc cookie keeps the fbclid so it survives the rest of the visit. Its value has a fixed format: a version, a subdomain index, the time it was created in milliseconds, and the fbclid itself.

Send the cookie's value as fbc with every server event. Do not change the fbclid part: Meta warns about a modified fbclid when the value does not match the click it recorded.

fb.1.1730700000000.IwAR2xYz...

_fbp: the browser id

The _fbp cookie is set by the Meta Pixel and identifies the browser across visits to your site. It has the same prefix, a creation time and a random number. Send it as fbp with server events.

fb.1.1730700000000.1234567890

Why they get lost

The pixel sets these cookies. If an ad blocker stops the pixel, it never writes _fbc, so the server event goes out without the click id and Meta may not credit the ad. Short cookie lifetimes in some browsers make it worse.

• Write _fbc yourself: Read fbclid from the landing URL and write the _fbc cookie from your own domain, without waiting for the pixel.
• Keep it on the lead: Store fbc on the lead record, so a conversion hours or days later still carries it.
• Pass it through checkout: If a payment happens later, carry fbc and fbp through the checkout so the payment event can send them.

How Unfunnel handles them

Unfunnel writes _fbc first-party on landing, not left to the pixel, keeps fbc, fbp and fbclid on every lead, sends them with every Conversions API event, and includes them in your lead webhooks.