Open-source vs. closed-source for centralised social-networks?

[Alec Taylor]

Fear open-sourcing fledgling social-networks; as centralisation is easily losable.

Open-sourcing social-networks when large, seems to work (e.g.: Reddit).

Without centralisation it becomes difficult to establish community. An example of a "decentralised" open-source social-network is: Disapora. Personally, I am using Python to build a niche social-network.

Is it more likely that open-sourcing a niche fledgling social-network would result in: many decentralised communities—or one [larger] centralised community?