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.
https://en.wikipedia.org/wiki/App.net
https://en.wikipedia.org/wiki/Path_(social_network)
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.