However they gone from PHP to their self hosted language where it will even be hard to find developers when Hack won't grow. Hack is close enough to PHP

However they gone from PHP to their self hosted language where it will even be hard to find developers when Hack won't grow. Hack is close enough to PHP where it's not hard to find devs who know 90% of the syntax right away -- and PHP syntax is quirky but not hard to learn either. Furthermore, developers don't need to have history with a specific language to be effective; hiring procedures at companies like Facebook and Google generally don't care what languages your background involved, except that you should be really familiar with some language. Ultimately Hack made sense because they had a company of folks who've been writing PHP at scale and at speed for a long time. They wanted the subset of PHP that wasn't so unusable, and to make it good and fast. They've largely achieved that. It definitely makes sense. And they do use other languages: Haskell, Erlang, JVM/C++ where needed.