Not necessarily a single big instance. Just one big implementation; that may use multiple instances under the hood. The centralization of Amazon allows it to polish up some kind of "portal" without worrying about the traffic concerns, and then use that to let users connect to internal instances, seamlessly, and external instances in common federated ways.

I know it seems like a long way around for not really providing any "value", but the idea is that most people just want "next twitter". They simply aren't going to deal with server instances in the way that, say, the gaming public does. People running businesses, being "influencers", or developing journalistic followings do not get any value from caring about technical implementation. So Amazon's strength, here, is that they could pick up a ton of influential people just by being "easy" and then the followers come for their interests.