I think the valid argument here would be that PHP is the cheapest/easiest option for most webmasters out there. When a normal person -- say an artist -- buys hosting, it isn't likely that he'll sit down and set up Python or Ruby. No, he'll upload a copy of Wordpress (on PHP) and be blogging in a couple of hours.
The reason Facebook hasn't migrated away from PHP is because it has incumbent inertia (it's what's there) and Facebook's engineers have managed to work around many of its flaws [...]
That said, it's NOT true that Facebook still uses PHP because they get cheap developer power. See this Quora thread: http://www.quora.com/Why-hasn-t-Facebook-migrated-away-from-...
From FB's former Director of Engineering:
The reason Facebook hasn't migrated away from PHP is because it has incumbent inertia (it's what's there) and Facebook's engineers have managed to work around many of its flaws [...]