There's also a far bigger pool of talent to draw from if you want developers. I don't think there are many people who know how to write web applications in Common Lisp or Haskell.
Let's be honest. 99% of the people that start a PHP project have it fail before it ever needs to scale. The people that do need to scale, though, are up a creek, because if you started your project with PHP, you probably don't know how to write a better VM for it. Consider, as proof, the fact that there is no better VM for PHP.
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.
You can get a web app up and running quickly in good languages too, and then you also don't have ongoing maintenance nightmares and the massively increased hardware and hosting costs that come with PHP.