You wouldn't believe the number of calls I got when I worked at an agency every day asking us to build facebook, but better in some tiny way. For free. The really generous prospective clients would offer us a share of the potential, expectedly limitless, profits. Critical mass of users is hard enough when launching real tools, social networks are the one in a million perfectly timed shots and follow up with great response to users' needs. You can't manufacture that kind of success, just hope to ride the wave well if and when it comes.