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

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.