> I don't know why, but psychologically I would feel like this is somehow less private than, say, a mailing list. I'm probably becoming old :)

I think that's a really interesting comment! Perhaps it's because the UX is more social network-y (FB/Twitter), which are historically public, and less email-y+historically private?

That's possible, but a more likely reference is forums. The ones I used were mostly public too :)