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.
