I've been thinking on and off about the peak and decline of text Usenet (http://www.cam.ac.uk/cs/newsserver/newsvolume.html) for a while now. While the easier moderation of blogs and web forums might indeed be part of it, I suspect that actually the greater ease of both creation and discovery is a big part of it too.

Creating a blog or web forum can take as little as seconds; creating a newsgroup in a managed hierarchy takes days a minimum and can take months in the worst case. Equally finding web forums is trivial (http://www.google.com/search?q=cycling+forum) and newsgroups don't tend to appear in the same lists. The ease of creation, plus time, is arguably part of the cause of the ease of discovery.

I think these things are, at least by now, primary and the better moderation features of the web secondary, because you don't get to be put off Usenet by the trolls and flamewars if you don't even get to see it in the first place. (Granted that this might have been less true ten years ago when a greater proportion of the online population would have been aware of Usenet.)