I have been thinking perhaps the idea can be carried further and we can separate the user-facing front end and the back end.
Imagine having multiple front end servers (e.g. fe1.site, fe2.site, ... fe5.site) all connecting to the same user database and the same back end server which serves the communities and contents etc (call it be.site for example). A user signs up once and can login to any front end server with the same account, create a community /c/whatever on e.g. fe3 and it will be accessible automatically on fe1-fe5.
This is in addition to the back end federating with outside servers. Outside sees the community as be.site/c/whatever and users there as be.site/u/whoever. (or maybe make an alias like www.site/c/whateverwww.site/u/whoever).
Additional front end servers can be added to spread the load if there are many users. If done right the users shouldn't even need to choose (or be aware of) which front end server they log on to, it can be automatically load-balanced. Another idea would be that special front end servers can be created to only serve API calls for apps.
I'm not sure if this will have bottleneck somewhere else, but I think this is an interesting idea to explore.