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Though it is internally-built, Hack is already open source at https://github.com/facebook/hhvm/.

FB uses a pretty wide array of languages internally -- I don't know if they release statistics publicly, but you can filter/search their open-source projects by language at https://opensource.fb.com/projects/#filter.

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I believe Hack is also used by a number of other companies, the most prominent being Slack [1]. FB probably uses almost every language in at least some context: Hack, C++, Python, Haskell, OCaml, Rust, JS, Obj-C, Swift, Java, Kotlin are all ones that I’ve heard referenced by friends who work there, though that’s likely nowhere near exhaustive.

[1] https://slack.engineering/hacklang-at-slack-a-better-php/

And Erlang (mainly in WhatsApp)

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