And because you are still reading and maybe plan to run your own instance. It's not “just” the tech or not “just” the social part. It's difficult and time-consuming work that isn't easily scalable. I know it’s a great feeling if people like what you build, but it wouldn’t help anyone if you burn out or your instance is going to be a hell because you have not the resources to moderate and administrate it properly.
If you still think “an own instance sounds like a great idea!”. Do it! But scale organic. Try to limit the registrations. Work with invites. I would recommend a closed registration and everyone with an account can invite more people. This worked great for a long time on my instance, and it prevents most of the problems above.