Yep. Zeus is a fork of Zookeeper, Hack is a fork of PHP, etc. It's usually needed to make it work with the internal environment.
The few things that don't have forks are usually the open source projects like React or PyTorch, but even those have some custom features added to make it work with FB internals.
Google also maintains a monorepo with "forks" of all software that they use. History diverges, but is occasionally synchronized for things like security updates etc.
Few companies experienced the explosive growth fb did, though many will claim to have done so. Hack made the existing codebase of php scale to insane levels while reaching escape velocity for the overall company to even attempt to transition away or shrink the php codebase, as i recall (i was an SRE, not a dev)
my memory is hazy, but when i started i was called an sre. Then someone made up the term "app ops". then it was production engineering. then I called myself an SRE when i was interviewing to leave fb.
Oh, and i was called a DBA before being acquired by google.
The few things that don't have forks are usually the open source projects like React or PyTorch, but even those have some custom features added to make it work with FB internals.