Hypothes.is is general purpose but has a focus on scholarship. They have different namespaces (including global) and work with different communities (and customers) to provide overlaid environments.

Moderation and curation are perennial issues. Maybe these ovelays will help. It's not the same, but Reddit has different communities on different subreddits (with different moderation policies) discussing the same thing.

And re portability, they've made some very encouraging noises about data portability and exhange. It's baked ito the standard, and their default content license is, I believe CC something.

Thanks for catching the typo!