I love PHP normally and I love it even more when it doesn't have syntax highlighting or indenting and isn't even displayed in a fixed-width font!
I love it too, but yeah they could do without. I wonder why they're not using PHP 5's OOP features. Could really help simplify things for them in the long run. Hell I'll be more than happy to refactor FB's source code to fit MVC for equity ha ha.
They do use PHP5's OOP features in Thrift. I'm guessing that the FaceBook code covers a long time period. The parts that have been leaked are core site functionality, which means they were probably written early on. PHP5 was still quite new and untested when FaceBook was developed in 2004.