I don't know what the exact deal is, but I assume Hack meets specific use cases at Facebook and can be put in production. It doesn't matter if anyone else is using it.

Maintaining a language meant for web servers is also objectively a much easier job than something like C++/Carbon which is supposed to be at system level, general purpose and has huge standard library.

By comparison, virtually nobody is using Carbon at Google for production code.

The counterexample, ironically, is Flow. It's practically irrelevant these days -- many (if not most) of Facebook's open source projects use typescript.