Because you can get a web application up and running quickly in PHP, then tune it later if and when your application needs to scale to 500 million users (which it probably won't, because only a handful of sites reach that size).
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.
Facebook really doesn't care what languages new hires know. Only a small number of people I started with had any experience with PHP. Anyone who works here is capable of becoming proficient in the language during bootcamp (http://www.quora.com/How-does-Facebook-Engineerings-Bootcamp...).
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.
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.
>Because you can get a web application up and running quickly in PHP
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.
I don't know where you're getting this "massively increased hardware and hosting costs" from - we run our trading platform on bog standard x86_64 hardware, as has every other PHP site I've worked on (some even run in VMs with 500MB of memory).
Because you are buying 10 times as many servers, and paying for power for 10 times as many servers. Where on earth did this "bog standard x86_64 hardware" business come from?
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.