Did you ever use Branch (since acquired and sunset by Facebook)? It was, in my opinion, one of the most thoughtfully designed social networks I've ever used.

In many ways, it was an evolution of Twitter, but structured from the ground up around respectful discussion. You had, from memory, 3-5x the character count for responses, and conversation threads were 'invite only' – anybody could watch a discussion play out, but would have to request permission from participants in order to participate. Those who weren't invited could simply 'branch' the conversation, in which case the original participants would not be notified – a perfect way to balance providing the ability to speak freely against targeted harassment.

It's a real shame they weren't able to take the concept further. I'd love to see somebody develop a similar service as my guess is that Twitter is now organisationally incapable of executing something like this.