[wott]

Yes, HN and Reddit are very-high-traffic forums, so it is mostly read-once--write-once--do-not-come-back. Not only the comment go by hundreds on each thread, but the threads themselves have a life expectancy of 24 hours only, 48 hours max, before they are buried and reject to the infamous page 2 where no one goes. So there's generally no point in coming back.

The fact that the 'shape' of the threads changes all the time does not help either (the way the messages are ordered, depending on the upvotes, the freshness, and whichever mysterious other criteria may be involved).

Conversations at a depth higher than 5 or 6 levels is also difficult to follow, so I think not many people read them, let alone take part in them (on Reddit, such deep threads even suffer a penalty twice, and the second one, which requires opening a new tab or window if you don't want to have to reload the whole page once you come back, is probably fatal to almost all of the readers).

(I won't tell how much I miss Usenet and newsreaders once more, I promise :-D )