>What if they're not scaled? What if scale is inherently constrained?

Very possible. I'm on Tildes and its invite only structure prevents the infamous Eternal September effect. It also means that it's nearing a decade and is very much not going to compete with other forums as a platform.

I'm perfectly fine with that. But that doesn't seem to be what people en masse want. They want to connect with all their friends and family, and discover new ones through specialized communities. On a scale of a billion people, that's hard to manage. And if no one principled fills that void, the unprincipled will.

>What kinds of constraints might form a path towards to this kind of small success today, in 2026, while there are giants like Meta stomping around?

Plenty of methods for that, centralized or decentralized. It's less a matter of "do we have the technology/ingenuity" and more "can it defeat the massive network effects?"