I don't mind Facebook crawling pages as long as it respects robots.txt, but for the last few weeks we've been hammered by requests from Facebook-owned IP addresses (millions of hits daily, 50+ for the same URL at times). They don't even set the User-Agent header.

There's a bug report regarding the missing header here: https://developers.facebook.com/bugs/1654459311255613/

Unfortunately it seems impossible to get in touch with Facebook devs directly.