which php framework is the best for building a social networking website

Personally I prefer CodeIgniter, it's fast, lightweight and has extensive documentation.

It also has plenty of community created plugins you can use to aid in development.

It's all personal preference though to be honest, there is no real 'best' framework, you just have to pick one which you can use to it's full potential. Play around with a few and see which ones you can get away with!

Best of luck!