Last I checked (2 years ago) Meta was using Sapling, a very heavily customised open source Mercurial frontend with proprietary backend.
FWIW the Sapling frontend can also be connected to a Git backend, and I've been using that for all my open source projects to get the best of Mercurial's user experience niceness while collaborating via GitHub <3
Still Sapling + Eden to make local work on a sparse checkout viable.