I used it for the last year or two at a company where all of our source control was with github. It has bindings to interact with github (not sure about what else) that allowed me to use it locally without anyone else having to change their workflows/install anything. Granted, I was mostly using it for stacked pr management among a few other things and not really fully taking advantage of all of it.

Yes, it uses Git as the default backend, so it's more or less just a different interface to a Git repository. Everyone today uses it this way.

The server-side components have been open source, but not fully usable due to fb-only code. That's changing and you can in theory build a working OSS server now that works on mysql/s3, but it isn't supported yet.