> This left me wondering if there is enough demand for smaller social networks with very limited visibility that can foster rich and insightful discussions (although this sounds like forums from the nineties). And if it's possible to keep the sales people away, or at least invite-only.
You can't keep sales people a.k.a phonies away it will always happen. Every underground subculture gets appropriated as soon as even one of these people shows up.
For example: https://en.wikipedia.org/wiki/History_of_the_punk_subculture
Imagine you run into Patti Smith and her friends in the 70s, if you're lame you won't be staying long. In fact you might get punched in the face. There is proper gatekeeping.
Once the scene grows even a little there's money to be made. A different sort shows up who understands the culture and likes the music OK but don't truly give a fuck it is about selling t-shirts and bootlegs for them. And/Or getting laid.
They'll start promoting it to grow it bigger and make more money. That's when the real sharks show up and the original members start leaving. Later still normies shop your style at the mall with no idea what any of it means or where it came from.
Sadly, can't punch people in the face over the internet. Even worse everything is all about forced inclusivity now.
This ain't a technological problem.