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New code at Facebook does use both. Instagram uses both extensively.

Does Facebook use Python 3 for Facebook frontend website? Or in PHP/Hack? (Facebook website frontend is well known to be in PHP/Hack)

Facebook's WWW is Hack. Instagram's WWW is Django on Python 3.6.

> Hack allows programmers to use both dynamic typing and static typing. This kind of a type system is called gradual typing...

I wasn't aware of such a type system, and there're many languages which use both:

You have links to public repos, or you're speaking from work experience?

I work at FB.