I wonder if there should be a visibility option which makes a post visible to everyone who follows either you or the person you're replying to, and possibly to everyone else who also responds. Then you'd just be talking to the other people in the room, so to speak, like you would in person, but not hiding your reply from people you might want to see it.

OTOH, the setting would undoubtedly confuse some proportion of people. Maybe a large one.