Creating a throwaway for this because I don't want to get anyone in trouble.
Today, right now, neither Facebook nor Google are fully centralized on Mercurial. Facebook uses a mixture of Subversion of Git, and Google uses a mixture of a system that is totally not Perforce why do you ask please do not look behind the curtain, and Git.
Moving forward, this statement will be true for both companies. In Facebook's case, they are narrowly focused on truly moving everything into Mercurial, and are heavily using Mercurial's extensibility to facilitate that. remotefilelog (https://bitbucket.org/facebook/remotefilelog) is a key part of this, but they've been doing lots of other stuff that's not public that I can't link to that lets them get further. This process is well underway, but (last I heard anything) is incomplete.
Google is going a different direction, because Google. They're aiming to extend out their not-Perforce system to speak scalable Mercurial protocols. Given that they have experience storing Mercurial and Git inside Subversion repositories on top of Bigtable, this actually strikes me as a relatively sane engineering decision, from their perspective. This project is not ready for use. Even when it is, public Google projects will continue living on GitHub, which means that Go will continue being Git for the foreseeable future (as will FOAM, Blaze, and anything else they want contributions on).
Today, right now, neither Facebook nor Google are fully centralized on Mercurial. Facebook uses a mixture of Subversion of Git, and Google uses a mixture of a system that is totally not Perforce why do you ask please do not look behind the curtain, and Git.
Moving forward, this statement will be true for both companies. In Facebook's case, they are narrowly focused on truly moving everything into Mercurial, and are heavily using Mercurial's extensibility to facilitate that. remotefilelog (https://bitbucket.org/facebook/remotefilelog) is a key part of this, but they've been doing lots of other stuff that's not public that I can't link to that lets them get further. This process is well underway, but (last I heard anything) is incomplete.
Google is going a different direction, because Google. They're aiming to extend out their not-Perforce system to speak scalable Mercurial protocols. Given that they have experience storing Mercurial and Git inside Subversion repositories on top of Bigtable, this actually strikes me as a relatively sane engineering decision, from their perspective. This project is not ready for use. Even when it is, public Google projects will continue living on GitHub, which means that Go will continue being Git for the foreseeable future (as will FOAM, Blaze, and anything else they want contributions on).