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Exactly.

Exactly what? The replies three works great on Sublevel on any mobile screen.

So you don't think Facebook comments work?

Facebook comments only allow one level of replies.

Yeah. Twitter could still do that.

Reddit apps seem to handle it pretty well. Touch the metadata for the comment to collapse the tree, or tap the text in the comment to interact with it.

I use a reddit app for X hours per day, works perfectly fine.

Twitter is basically reddit without subreddits.

And (well defined starting points of) topics/threads.

I use reddit a lot, and I use twitter as little as possible because I hate the experience of using it.