I don’t work there, but from talks I’ve gone to I believe their Scaled Abuse team uses (or at least used to use) Haskell[1] along with the library Haxl[2]
It also powers the backend of Wit.ai which FB owns. Wit's open-source entity parser, duckling, is written entirely in Haskell. https://github.com/facebook/duckling
My guess is that most people using Haskell (and OCaml and Standard ML) are using it for internal tool such as program verification/analysis/transformation. It's not that Haskell/OCaml/Standard ML should not be used for backend services (i.e. APIs in a hot spot [at faceboook's scale anyway]). It's just that (in my experience) they're most commonly used for out-of-band analysis or developer lifecycle systems.