The fact that you never grew up in the social media world highlights exactly why you don't understand the problem despite thinking you do. Your good old "social circles being hard" issue is not the problem here.
Please understand that there are plenty of situations where someone gets invited to something if and only they are visible to others and easy to invite. i.e. there exist plenty of situations where being on the platform is the sole determining factor. And that being off the platform that the majority use puts a very significant, ongoing, and asymmetric burden on the host (or invitor, if it's a different person) to keep them posted on all the details that did not exist otherwise, and this fundamentally makes one less likely to be included, in an entirely natural and unavoidable fashion that is no fault of anyone involved, and has nothing to do with "social circles being hard".
You have to recognize when the system is making a problem worse than it naturally is.
