The one thing I'd add to this (excellent) piece is that the stuff he lists as important to social networks is necessary, but not at all sufficient. Copying all of the details of reddit is not going to get you a Conde Nast buyout in a year, even if you took the same route they did, copying every detail. The sociological variables are the hard part of making a site take off. You need to get an audience, you need to hold them, and you need to have them attract other people ad infinitum if you want to be judged successful.