Normally, bright developers acquire a new language* within a few weeks, if they already know the paradigm from another language. Hack is OOP, and so are many others, so I would expect a bright developer to learn hack in 2-3 weeks, not months.
Heck, I just read the docs of hack for 2 hours, and I already got a good sense of what the language can do. I worked with PHP internals, and a bunch of other languages in my career.
* as language we define the language, its syntax and semantics, not any other library or the ecosystem. And yes, the standard library is still just a library. Knowing a list of functions and parameters does not mean knowing a language.
Well Ruby is OOP, is it gonna be super easy for me to get a java web development job? All things being equal a hiring company will go for someone with 3 years java experience over my 8 years Ruby experience.
Facebook has no choice since no one knows their language, but most other companies will hire someone with the particular experience of their stack if possible.
The point being, you can hire good php devs.