Federation is critical to the development of alternative social networks. The reason people use specific social networks is because other people are on them. If there's an open source social network app that isn't federated, then everyone using sites powered by that app is isolated. With federation, all instances of the social network can interact with each other. Different instances can pop in and out of existence and the ones that stick around can become popular.

It's better for new social networks to be federated then isolated.