I would suggest an idea review - limiting the number of connections for a free account - this won't fly. You are solving the problem, that is well known in the tech world, but you are not solving problems of average users.
Edit: Aslo, why not just piggy back on GNUSocial/status/pumpio/etc.? Having a node that can be federated if/when the owner (i.e. you) wants to seems more scalable.
Either invent some non-intrusive advertising method or go after CPM. Paying for the social network in 90% will fail in the current environment. If you want some free counselling from marketing veteran, hit me up.