Most of Google. Some projects like Android and Chromium are developed outside google3 and use Git (with Gerrit).

IIRC, Git is also relatively popular as a local Piper frontend.

Mononoke/EdenFS (mentioned in Sapling's Github README) are Meta's in-house mercurial stack. The "Sapling" name is not used internally but "hg" is.

Facebook have been the only major users of Mercurial for years. Sapling is basically their rewrite of it.