Facebook didn't do it with "crusty old PHP;" rather, they had to re-build their stack completely from scratch to keep up. See HipHop, their homegrown PHP-to-C compiler https://github.com/facebook/hiphop-php/wiki/ and Cassandra, their own custom database system http://cassandra.apache.org/
If they stuck with crusty old PHP, I have no doubt they would never be able to manage the load.
No kidding. They're using php as a template language to call thrift services. That's hardly "using php" in the sense that most people would think of it.
It depends on how many servers you are willing to run. When you have 500 million users, and a decent amount of them, accesses your site multiple times a day, CPU cycles per request starts to count.
If they stuck with crusty old PHP, I have no doubt they would never be able to manage the load.