It's a toolkit for building communities. People will join because the communities they want to participate in are there (which eliminates the supposed dilemma of "which instance should I join?" - this is only a problem for people fleeing Twitter and looking for safe harbor, not for people discovering it organically). They'll stay because it's a good toolkit and they want to join more communities.
Or those things won't turn out to be true and it will fade away, nothing is guaranteed, but if you judge it based on whether it is Twitter, you're missing the value proposition.
In my view, having everyone in the same tent is one of the mistakes Twitter made; it doesn't need to be replicated. It created opportunities for messages intended for a certain community at a certain time to be taken out of that context and misinterpreted to fuel flamewars. It made it possible for there to be this spectacle of the "main character" who was ridiculed for day and then tossed aside.