I think this is a tiny number of people, but even then, how would an early username or URL on a hypothetically successful new social network benefit you anyway?

I think this is a tiny number of people, but even then, how would an early username or URL on a hypothetically successful new social network benefit you anyway? Unless you are already a "big deal" yourself for other reasons, a particular username on a social network isn't going to turn you in to one. Whatever one might think about social network "influencers", their influence is ultimately rooted in something about them (looks, charisma, knowledge, marketing dept strategy), not their username.