Which makes sense. Facebook now is big enough to sustain a programming language on its own. And for Hack, I would assume it mainly to replace where the PHP used

Which makes sense. Facebook now is big enough to sustain a programming language on its own. And for Hack, I would assume it mainly to replace where the PHP used to be, which is a shallow API composition layer, no heavy lifting stuff. To fulfill that purpose alone, one need not to have super diverse ecosystem providing all kinds of libraries, just focus on a limited core functionality should suffice.