Timing plays a role into this too. If you're interested in getting into a discussion on a submission, you need to check the site frequently because after a few hours the discussion is going to be dead. Likewise, you need to make the comment fairly quickly instead of waiting a day or two to chime in. And after making a comment, there's pressure to check frequently to see if you got replies - again, if you wait for too long nobody is going to notice your reply back to them.

With the old web forums you could wait a day or two to respond or a day or two to check for replies. But the timing dynamic here (and on many other sites) encourages much more frequent checks and much more rapid-fire conversations.