My impression is that Facebook has plenty of people who would not find learning Haskell, OCaml or Lisp particularly difficult, and would thereafter be more prod

My impression is that Facebook has plenty of people who would not find learning Haskell, OCaml or Lisp particularly difficult, and would thereafter be more productive than in PHP. In fact, I know for a fact that some of their internal tools are written in Haskell and some variant of ML. If there is any company that should not be worried about a language's barrier to entry or trying to get cheap programmers, it's Facebook--my impression is that they have an engineering team more like Google than your favorite nameless corporation. I think what Facebook is doing is working with a ton of legacy PHP code. In fact, from what I've heard, they're definitely open to using other languages in new projects; it's just that rewriting all the PHP code they already have would be horribly wasteful.