Facebook uses a heavily modified fork of Mercurial which they have (as of last year) released publicly, and now refer to it by a new name, "Sapling" (though the

Facebook uses a heavily modified fork of Mercurial which they have (as of last year) released publicly, and now refer to it by a new name, "Sapling" (though the command may still be called 'hg' internally). And Sapling actually stores data using Git's on-disk format, along with an entirely bespoke server-side backend. It is an entirely different project from Mercurial, now.