It wasn't the right tool for Facebook - Python would have been a better alternative, at least once they graduated from shared hosting. It was good enough that migration was never worthwhile, because the technology wasn't the innovative part of Facebook, but that doesn't make it good.
Just that Python is a better language for solving those kind of problems. I wrote a vaguely-facebook-like webapp in the same timeframe; later I worked (briefly) at last.fm (which uses, or at least used, the facebook stack) and spent most of my time working on better replacements for PHP code and issues that were caused primarily by PHP. If nothing else look at all the tools facebook produces - thrift and HHVM being the most prominent ones - to work around their PHP problems.
PHP was the "right tool" in that it was the language Zuck knew when it was a one-man project. And facebook's success is a reminder that second-tier technologies can be perfectly adequate to build a successful business on, that business concerns often have a lot more impact than technical ones. It shouldn't be taken as an indication that all their technical decisions were right.
