I have worked on a somewhat large-ish backend that was built with Hack. In the end, we dropped it in favor of PHP7, due to all the small incompatibilities it had, and all the non-standard things we had to do to use it. With PHP7 the performance difference was negligible (our original reason for choosing it), and the type annotations of PHP7 were good enough for us.

With switching, we were also able to use phan[0] and other tools that helped us write better PHP code that were previously unusable due to Hack.

[0]: https://github.com/phan/phan