It's not just what users are used to, it's what users are used to for that context. So if it's publish/consume content (even if some is user-generated), then n

It's not just what users are used to, it's what users are used to for that context. So if it's publish/consume content (even if some is user-generated), then newest should go on top because users are used to that. If it's synchronous two-way communication, then newest should go on the bottom, because users are used to that.