I'm sure there are many more that fit into the timeline, but they're all pretty much the same thing - an avenue for 5% of the people to have serious discussion and 95% of the people to be idiots and attention whores.
Exactly! Reddit is Usenet, except good stuff flows to the top and spam is eliminated through community voting and moderation, which is an improvement and no small one either. Plus it's easier for the non-techies to create/customize subreddits with their own subculture, so it can evolve and keep people engaged. So far, Reddit has scaled far better than Usenet did.
If you're talking about eternal September and the decline of quality you might want to look at forums with heavy handed moderation or a paying userbase. For example the Something Awful forums have a pretty good signal to noise raito.
One day we'll discover that arguing about random crap with complete strangers via our keyboards actually isn't all that interesting, and rediscover the world outside.
This has all happened before and will happen again.