WHY FACEBOOK DOESN’T USE GIT

Summary from the TLDR newsletter

Mercurial is a tool for creating and managing code changes used at Facebook. Facebook started out on Git, but it began hitting scaling limits in around 2012. Mercurial's developers were receptive to the idea of Facebook engineers contributing patches to scale it better while Git's developers were not. There were other alternatives, but all were quickly disqualified. Facebook adopted Mercurial not because it had better technology, but because the maintainers and codebase felt more open.