Have you ever noticed how email exchanges where it's just you writing back with a friend or two always seems to peter out after about 3 volleys? And how online discussions seem to go on and on? And how a lot of virtual communities--particularly the really active ones--are full of drivel?
Yeah, me too. I don't have a particularly good answer for it either. I think the sociological/psychological reasons are pretty straight forward: the conversations linger because the medium is time-shifted, each party of the conversation has to contribute less resources to keeping the discussion going. This is why flame wars are/were such a big issue on listservs and usenet, or at least part of the reason.
This isn't to say that flame wars aren't still an issue on the internet, or that usenet/listservs aren't still important, but I think a lot of the discussions that happen on the interent happen elsewhere these days. Usenet and group email, are still prevalent, but I think the inhabitants of both tend to be hacker types or old timers (and more likely both), and these are two groups who are on the whole pretty adept at how these kinds of communications work.
In the last five to ten years communications on the interent have really changed. We've gone from very structured mediums that come with built in threading, topically segregated conversation, with some semblance of push functionality and even web-based discussion forums, to unstructured and chaotic mediums like blogs, blog comments, and wikis.
While I am a bit nostalgic, the truth is that these changes have enhanced the ways that we can collaborate and really democratized the internet, and this is unequivocally a good thing. At the same time, as I visit really active conversations in threads or prolific ravelry, my brain hurts. There's too much to follow, too much noise not enough signal, and the strict chronological display changes the way that people read and respond non-productively (and probably is the root of most of my complaints).
There have been a number of "hacks" that various communities have deployed to help keep conversations together. Threading blog comments, or including comment numbers makes the comments on a given post easier to read help, but they don't change the fact that conversations on the new internet are really hard to organize and orchestrate, and after a certain point, enter.
I don't want to be a hipster and say "no one has genuine conversations on the internet anymore," but I think its really clear that the kinds of conversations that happen online (these days) are very constrained by the kinds of technology that are used to convey those conversations both in terms of how conversations develop, but also in terms of who can and does participate in those conversations. While this is in and of itself not a particularly striking revelation, I think it's also clear that we--as citizens of the internet--cannot just say "it's the internet anyone can access it," and let it go at that.
Sorry if this is a bit disjointed, hopefully I'll figure more things out in a bit.
Onward and Upward!