Their internal Phabricator is quite different from the open source version. When I worked there (around 2017), it was definitely miles ahead of anything I'd used before. A lot of how it works is probably... controversial, I guess, but it suited my mental model quite well.
I used Phab at a job and it was a complete mess. Stacked Diffs only sort-of worked, CI integration was bad, notifications were so noisy everyone tuned them out, etc.
Talking with a Meta person, it sounds like Phab really needs Mercurial to work well, at least for Stacked Diffs because you need to be able to identify commits independent of their location in history to properly maintain the Stacked Diff associations.
I wonder whether they'll continue using their in-house Phrabricator or choose to support Phorge (https://we.phorge.it/) now that open-source Phabricator is no longer supported.
Internal Phabricator isn't even really Phabricator anymore. In fact, the name changed to just Diffs. I'm pretty sure its entire codebase has been rewritten, at the very least into Hack from PHP. They have a custom API; not Conduit. Etc.
So, no. They wont support Phorge. They really never supported open-source Phabricator after Evan left and made it his own with Phacility.
reviewstack is the thing now, adds some parts of phab UX ontop of github apis. Been enjoying using it for oss stuff oursite meta and fixed a limitation of it recently, https://reviewstack.dev/facebook/sapling/pull/656 shows what it looks like with the versioning available
I used it briefly but couldn't get most people to adopt it widely enough.