Why does Phabricator's own site (phabricator.org) link to their own source on GitHub? If Phabricator is great for code hosting, shouldn't they be hosting their own code with it?
We link to GitHub because they have more infrastructure than we do for handling heavy read loads.
Generally speaking, we proxy requests and pass them through to the VCS after performing authentication and broad permission checks. Requests are rewritten slightly to remap user-facing display paths to actual paths on disk. Some additional features act through commit hooks.