I agree with Lexi. Even though I probably spend more time than I should online, sometimes a lot of interesting topics are brought up when I'm away at classes for a few hours or late at night on one of those rare occasions when I went to bed early. I think that as long as you quote the comment you wanted to discuss if it was posted more than a page or so ago when you're making a catch-up post, you're fine.
"There was this one time an alarm went off - it was in the middle of '24.' That's just wrong. Everyone knows everyone was watching '24' at that time."
--NYU student quoted in the Washington Square News about students ignoring the frequent fire alarms in dorms