Isn't that what you want for a Facebook replacement though?
I guess being able to browse public profiles and posts would be good, but if it's like Facebook then wouldn't most people be using their real names and posting about semi-personal IRL stuff?
You'd want that stuff to only show for your friends, and maybe friends of friends

No I think you're right, give them an instance. They won't have enough knowledge to choose an instance anyways. If they don't like the one you gave them then they can move later.