Is Facebook still using mercurial? It seems that there was a blog post about it in 2014, but their repo[0] just seems to say that their codebase was originally based on/evolved from mercurial.
Losing Facebook as a contributor and community member is the larger loss, regardless of how much their internal vcs resembles or integrates with mercurial.
[0] https://github.com/facebookexperimental/eden