I'm sure there are a few teams that use Mercurial incidentally somewhere, but our primary megarepo is all on a VCS called Piper. Piper has a Perforce-y interface and there are experimental efforts to use Mercurial with Piper. Also mentioned in the article below, there's limited interop with a Git client.
If you're curious what it all ends up looking like, read this article. It's a fairly good overview and reasonably up to date.
IIRC, the git client is deprecated; the mercurial one is meant to replace it for the use cases where you would want a DVCS client interfacing with Piper in the first place.
It's not deprecated. I use git-multi every day, much to the chagrin of my reviewers. Google thinks that long DIFFBASE chains are weird and exotic, and Google doesn't like weird and exotic as a rule.
