I'm thinking of building a social networking site in php. Does anyone have any advice? Do you think it is possible? I want it to have a user login and basic messaging service with profiles.
Regardless of your budget you'll struggle to compete with the big boys so I'd say try and find a niche, create a social networking site around a subject or hobby (something addictive). Like sami said its possible and I'd look into drupal as a base rather than hard coding the whole thing your self although you'll have to create some modules your self you should be able to find quite alot of pre made modules to help you. Here's an example of a social networking site made with drupal: http://www.betternetworker.com/
and i'll say regardless of the niche.. its money and effort that counts most.. when shawn started dp.. there were still big guns in the webmaster niche but he was able to compete.. The niche advice is very much fair but i dont think of any good famous niche nowadays which isnt saturated !