Facebook probably has millions of lines of PHP code, PHP code proven to work. This isn't just Facebook.com, but internal websites, their bug tracker software (that is open source), analytical software for PHP, etc. etc. Rewriting all that would be _expensive_.
Running it on normal Zend PHP, based on the benchmarks they get on HipHop, requires _way_ more hardware at the scale of Facebook, so again, very expensive.
PHP developers are abundant, PHP is a relatively easy language to teach, resources are everywhere. Due to their abundance, PHP developers (even at Facebook quality) are probably less expensive than other languages. Easier hiring and lower cost developers, cheaper.
If you read the comments, you'll find the answer further down. They do address it. At their scale, all languages/platforms break and they would need to invest the same amount of time and effort getting it to work.
It won't make a difference if you switch to the latest hipster language. You'll still need an effort of this magnitude to get it to scale the way they need it to.
Furthermore, rewriting an entire code-base is a monumental undertaking that is both expensive and complex. Especially for something as large and diverse as Facebook. It also makes zero business sense and Facebook is a business.
Why should they take a risk with a new code-base, when what they're doing is clearly working? PHP just works.
Changing languages in the scale of Facebook, means that everyone, internal or external, need to know the new language.
This transition is a big process, which usually costs much more money in trainings and loss of productivity, than having a dedicated team improving the performance of existing tools.
No I'm genuinely asking. Isn't some of the stuff here already being done by other languages, or is Facebook really breaking new ground here? (yes lbrandy I saw your earlier comment)