alt Hacker NewsIIRC Facebook switched to HG from SVN in the 2010s, one (main?) motivator being that the single repo was getting too big and svn’s only way was to start splitting it up. Which was against the philosophy of openness of the single repo. No idea what’s Meta doing now.
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