I would add that social networks can fail or succeed depending on whether they attract the right type of crowd at the first place, then balance members quality and quantity. Example: if you and your friends joined a social network, only to find out that the coolest kids in your class are on another one, will you switch? Most likely because cool people will attract more cool people. This networking effect is behind the success of Linkedin, who has many members at VP level and above. So if I were to look for a job, would I go to the place where VPs hang out or just any social network as long as it has loads of functionality?