Many people probably left when better file sharing methods showed up. I remember all that, before bittorrent was even a dream, when you'd have a bunch of fserves instituting their own caps and queues and stuff on dozens of different mostly-content-specific servers. You'd have to navigate all those insane directory trees trying to find stuff and make drug deals to get the "good" files. Ah, the good old days.
The scariest thing to me is that I'm not even that old, and yet so many people these days don't even know what IRC is. It's like Usenet or something. I think I did a good job of loosely applying this monologue to your reply :P
In the same vein, all forum software is a weak reimplementation of Usenet. Not distributed, no killfiles, can't pick your own client interface, can't make your own groups.
I learned a valuable lesson when I watched AIM use explode in high school. I realized that non-geeks started using a crappy chat interface because it was well marketed and ubiquitous (AOL and AIM). The same thing happens with many inferior technologies. Java is an extremely weak reimplementation of some most fundamental semantics of Lisp (such as garbage collection), and, thanks to marketing, became exceedingly popular. Twitter is the same thing (except I still don't see non-geeks using it).
Fundamental to IRC is the concept of rooms. Twitter is a web of one to many relationships.
"Conversations" on Twitter are much more segmented and have less depth. I see the Twitter concept as a much more broadcast oriented sort of thing rather than a real conversation facilitator.
The most disappointing thing is that IRC is still awesome and bots make it infinitely more useful than a webapp could ever be. Unless I can setup Campfire bots or something.