Edit: And for those that are just shocked that git isn't the answer.
Facebook: https://code.facebook.com/posts/218678814984400/scaling-merc...
Google: http://www.primordia.com/blog/2010/01/23/why-google-uses-mer...
Mercurial was pushed internally as being the "better" (for some dimension of better) between it and Git back in 2010, but I think even the most hardline Mercurial fans have realized that in order to meet developers in the middle in 2015, we need to use Git for our open-source releases. We have a large investment in Gerrit [1] and Github [2] now.
So the Mercurial comment is probably entirely based on scaling and replacement for the Piper Perforce API, rather than anything externally facing.
[1] https://www.gerritcodereview.com/ [2] https://github.com/google
https://selenic.com/hg/log?rev=@google.com&revcount=200
I always see a lot of Google and Facebook at the Mercurial sprints.
But if I were to take a poll of the informed and opinionated engineers sitting around me, they would almost all likely take git over hg.
In any case, git is in actual widespread use _now_ at Google, for Android and Chrome, and other open source stuff (some of which was moved recently from code.google.com to github).
The commands are still basically the same as Google's wrappers around Perforce and the learning curve was non-existent for someone who was used to using Perforce at Google.
One thing mentioned in the paper, but not mentioned here, is that there are teams that live in GIT, such as android and chrome, but they are not monolithic.