I'm a lot less concerned about UX/onboarding issues than about the fact that the simplistic federation model is going to fall apart long before they even reach

I'm a lot less concerned about UX/onboarding issues than about the fact that the simplistic federation model is going to fall apart long before they even reach 1/10 of Twitter's scale. Now, if you're OK with having a lot of smaller communities that only communicate sporadically, forcing people to have ten identities and ten tabs open to participate effectively in each one, that's not a problem. However, if you want to have a single identity fully participating in multiple communities, then the federation model needs to improve a lot. The current ad hoc relays are just the first step on a long road toward a network/proxy/cache layer that can reduce the unnecessary latency and load on the servers themselves, to provide a federated experience beyond the old FIDOnet level. There are lessons to be learned from IRC, Usenet, and P2P file sharing that can help with this.