There just needs to be automatic instance assignment for new users with registration time opt-out for those who want to choose. Pick whichever one is the most populated. If that’s down at that particular moment in time, pick the next most populated and so on. Take the instance picking out of the equation for people who don’t care about that. People who do care can still pick their own.
That being said, it’s difficult to grow a new website in general unless it’s already got established backing from other source (i.e. a large corporation, celebrity, etc.), It’s not like old days in the 90s when there were far fewer websites in general.
He’s one of the best of all time, without question, IMO.