Yes, there is no gigantic pool of experienced Haskell or Common Lisp developers. However, I suspect Facebook has a disproportionate amount of them. Additionall

Yes, there is no gigantic pool of experienced Haskell or Common Lisp developers. However, I suspect Facebook has a disproportionate amount of them. Additionally, anybody working at Facebook is very likely able to pick up Lisp or Haskell relatively easily even if they do not know it already. Facebook is exactly the sort of company that does not look for developers of a particular language. I know a bunch of people who have interned there and several (ones a year or two ahead of me) who are going to work there full time. None of them had any PHP experience. Some of them still don't (the one I know best used C++ there, if I remember correctly). I think the PHP at Facebook is nothing more than a legacy of how it started.