I would go with PHP as it is community driven development and as it goes most of things will be free and coming numerous varieties where in ASP you more likely to end up paying for this or that. Secondly I find ASP slower in comparison to PHP or JSP.

Alot of people throw this quote about but there is no reason ASP.net websites shouldn't be as fast (or faster) than php sites. It simply comes down to coding practices and optomizing performance.

I would consider what level of interaction/features your users will require. As persoannyl i find it easier to build a "Richer" interfcae using .Net rather than PHP. But again there is no reason why this should be the case. But i do agree with peter_budo...
that a social networking site would more than likely work best in a community driven environment

At the moment i would go with php purley because of the huge scale asprox botnet.

Also, call me crazy but i prefer to code in php.

your crazy