To elaborate on a comment I left earlier.
There is no flow of discussion in the typical case -- comments always address the person "on the podium" (aka the post owner) unless they explicitly indicate otherwise. In other words, every post is like a little blog entry or presentation, owned by the person who wrote it. Like so.
During a presentation, when someone raises their hand in the audience and asks a question, it is quite safe to assume they almost neverare not addressing another random audience member.