When Meta matches your conversions to users and ads, it relies among other things on two cookies: _fbp and _fbc. For server-side measurement and the Conversions API, it's important to understand them, because they help raise matching quality.
_fbp (Facebook Browser ID)
_fbp is a first-party cookie set by the Meta Pixel. It identifies the browser and serves as one of the signals for matching events to a user. It's created on visiting the site, regardless of where the visitor came from.
_fbc (Facebook Click ID)
_fbc is derived from the fbclid parameter Meta adds to the URL when someone clicks your ad. So it ties a conversion specifically to an ad click — a key signal for attribution. If the visitor didn't come from an ad, _fbc may not exist.
Why to send them via the Conversions API
In server-side measurement via the Conversions API, _fbp and _fbc are sent in user_data together with other identifiers (hashed email, phone). The more quality signals Meta receives, the better the Event Match Quality and the more accurate the attribution. Omitting fbc means losing the direct link to the ad.
Things to watch
- Capture
fbclidon arrival from an ad and store it, so you can send_fbcwith a later conversion too. - Send
_fbpand_fbctogether with hashed personal identifiers — they complement each other. - As with all personal data, send it with consent.
How these signals fit into Meta CAPI setup is covered in the complete guide to the Meta Conversions API. When the match rate is low, the guide to raising Event Match Quality helps.