facebookexternalhit

Operated by Meta · Security and integrity checks

If you block it

Links to your site shared on Facebook or Instagram lose their preview card — no title, no image.

Fetches links shared into Meta’s apps so it can cache the title, description and thumbnail.

robots.txt: Says robots.txt may not apply“might bypass robots.txt when performing security or integrity checks”
robots.txt caching: Meta says changes take up to 24 hours to take effect, because its crawlers use a cached copy.
User agent: Not published as a full string. Meta publishes only the token.Match on the prefix facebookexternalhit rather than the whole string: operators version these, and an exact match stops working on the next release.
Verification: No published method.Meta recommends allowing crawlers by IP address, calling it more secure — but publishes no IP list and no reverse-DNS method for any of its five crawlers.