Judging by the fact that most of Facebook is based on PHP, it sounds to me like it's pretty robust... It's also object oriented.
The only drawback that I would find as a code-geek is weak typing
Which Facebook is addressing with their language Hack, which is heavily based on the PHP language. As a bonus, you also get a language that's built from the ground-up to be fully functional with their HHVM technology. It's an exciting time for PHP.