I suspect a lot of people here are the ones in their circle who bring in a lot of the cool info that their friends missed out on. This still sounds like Slack.
We're talking about webs of trust aren't we? Not about chat rooms.
I'm hypothesising that any such large scale structure will be perverted by commercial interests, while having multiple Dunbar sized such structures will have a chance to be useful.
How about restricting them to everyone-knows-everyone sized groups, of like a couple hundred people?
One can be a member of multiple groups so you're not actually limited. But the groups will be small enough to self regulate.