Facebook has been using their own in-house Sapling/Eden for years and years now. I'm not sure how much similarity remains with open source Mercurial.

You can use all the normal hg commands at Meta. It’s a regular mercurial repo from the standpoint of a user

Yeah, but people using open source mercurial aren't in the same boat. It's a different codebase.

Facebook didn’t adopt mercurial until something like 2014? And before that used svn -> git