Our engineers were comfortable with Git and we preferred to stay with a familiar tool, so we took a long, hard look at improving it to work at scale. After much deliberation, we concluded that Git's internals would be difficult to work with for an ambitious scaling project.
Instead, we chose to improve Mercurial.
Together, the hgwatchman and remotefilelog extensions have improved source control performance for our developers, allowing them to spend more time getting stuff done instead of waiting for their tools.
How many other organizations with large codebases will follow Facebook to drink the Mercurial water too? Facebook's move to Mercurial should come as good news for Bitbucket as well.
Mercurial or Git? Tell us on Twitter.