In my view, Facebook's success isn't proof that PHP is awesome. It's proof that they hired awesome people.
The groupthink MySQL/PHP hate here implies these are terrible products which only a moron with bad taste would use, which is demonstrably false when you look at the choices made by those using them.
As far as I'm concerned, PHP is the Bud Light of programming languages. Popular and perfectly adequate for a large audience, but definitely not a sign of taste and discernment.