Both Facebook and Google are continuing to contribute to Mercurial, so they both have some vested interest in it. If you poke around the commits on the repo[0] you'll see commits from people with @fb.com and @google.com email addresses. The mailing lists also has activity from both companies still.
As well, the Mercurial team does quarterly sprints (I believe), and Google is hosting the next one[1].
As well, the Mercurial team does quarterly sprints (I believe), and Google is hosting the next one[1].
[0] https://www.mercurial-scm.org/repo/hg
[1] https://www.mercurial-scm.org/wiki/4.2sprint