> The UX is important. It's important that all users have the same one. For link aggregators, you want comments displayed as a tree. For microblogging, flat lists work better, especially when people post "threads", which they do.

That's subjective and exactly the type of thing that could be handled in a client. In both cases you're displaying the exact same data. A microblogging thread may be displayed as a flat list, but it's still a tree like a link aggregator thread.

> I meant a Facebook-style social network. So a platform to post what are essentially blog posts. In ActivityPub, that would be the Article type, which, according to the spec, has no semantic difference from Note. An Article could be created as a Note and vice versa with no semantic difference or loss of data. Just like above, the data is the same and the only distinction is the visual presentation of it.