[ies7]

Google and Facebook use mercurial over git internally.

[dcgudeman]

Google doesn't use mercurial they use perforce.

[surajrmal]

Google uses a proprietary backend with the primary front end written to look like a clone of the perforce tool p4. The newer frontend that's getting investment is mercurial based. There also exists unofficial/deprecated support for a git frontend.

[loosescrews]

I believe Dropbox uses Mercurial as well.