If the people you find to write code aren't capable of learning something other than PHP, you should find new people. People who write code for Facebook should be good at programming, not just "good" at PHP.
But the language itself is conducive to writing a lot of code rather quickly. There is no memory management to worry about, there isn't a bunch of verbose boiler plate like in Java. You just write code and even if it's not perfect, you can iterate on it quickly.
That's not to say that's how I'd run Facebook given the opportunity, but from talking with people who have presented HipHop here, these are the reasons they give.
My understanding is that Facebook rarely hires PHP developers, it hires other developers and teaches them PHP.
This bases on a conversation from 2009 when they had just 200 programmers, so this is likely to no longer be true. However, I think it's highly likely that Facebook developers know a lot more than PHP.
