The software part is easy. The hassles are more in dealing with abuse, ddos attacks, inappropriate content, etc. Hosting can also get expensive if usage gets high enough. But, things like this have been done plenty of times and there is lots of such software already available.

How well do you think the federation model is working, in terms of cultural dynamics, more defederations than I would have expected to see, etc.? I’m not counting technical glitches that I assume will get sorted out over time.