What's your motivation, aside from modernizing Facebook's code base? What language niche does Hack serve which is not served by other languages? Why Facebook at all (aside from the rarity of finding a company to pay you to write a compiler :))?
This lets you choose the pace and extent to which you want to adopt the safety of static typing, while preserving your dynamically typed code -- and without sacrificing the rapid turnaround of PHP.
That's a unique combination, in my experience.
Typed Racket also includes more types, like ": Integer [more precisely: Negative-Fixnum]" (from docs), as well as polymorphic data structures and higher-order functions. I don't know if this is a "fundamental difference" but it might mean Facebook's type checker can optimize in certain ways that Racket's cannot.
The Typed Racket type checker is much slower than Hack, though.
Hack does seem to have polymorphic data structures and higher-order functions.