It's a shame that Dropbox abandoned Mercurial for Git. With both Facebook and Google contributing to better support for monorepos in Mercurial, Mercurial seems like a better choice for big monorepos.
Huh? I don’t know about Facebook but Google for sure does not use Mercurial as monorepo backend (nor does it use Git). There are Git and Mercurial-alike clients to interface with the in-house backend which is a Perforce like thing. Neither Git nor Mercurial would be fun to use at Google scale. Dropbox has a much smaller monorepo hence they can clone the whole Git thing on developer machines. I assume doing the same with Mercurial is impractical as no one has patience for something that slow.
Facebook has a Mercurial clone named EdenSCM[1], which includes a scalable backend named Mononoke[2].