Facebook does use Java (entire Hadoop infrastructure, HBase deployments, likely more), Haskell (AST manipulation, afaik), OCaml (with a fairly interesting libra

Facebook does use Java (entire Hadoop infrastructure, HBase deployments, likely more), Haskell (AST manipulation, afaik), OCaml (with a fairly interesting library for static analysis: https://github.com/facebook/pfff https://github.com/facebook/pfff ), C++, and C (fairly self explanatory).