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/