Also, this is more about imposing a qualitative filter on the users who sign up. If the earliest users you have don't like you enough to give you their social login details (assuming, of course, you don't do something silly that poses a security threat) then do they really like you enough?
I like to think of my earliest users as "apostles" (I even call them that. Not to their face, but yeah) and they should have a bit of faith in the service. If they don't, then they are not really apostles after all.