One of the most fascinating past of Web 2.0 for me, is the way it paints a study in crowd behavior. When I think about 2.0 sites, one of the biggest differentiating factors is their dynamic nature - where the readers contribute as much if not more than the site itself. And the tone of this contribution is distinct, an accent if you will, of these sites. Consider the following examples:
Slashdot, as the site proclaims, is a site that provides news for nerds and stuff that matters. The site for long has defined a sort of intellectual nerdy sub-culture on the Internet. Before the 2.0 moniker became the fad it is, Slashdot derived more from the comments everyone posted on "nerdy" news stories, than the stories themselves. And the tone on the comments has always been something that defined the site - nerdy, important, focused on being right and mostly brutal and unforgiving.
Woot is a hoot. For an e-commerce site that sells one - just one - item each day at a ridiculously low price, Woot has developed a strong following of users who go out of the way to research each deal. The tone starts with the description that is posted with each deal. Rarely focusing on the subject of the sale, the description is funny, satirical and whimsical. A tone that follows throughout the site into the comments. In stark contrast to Slashdot, your head is now chewed off if you are wrong. User posts are creative, sardonic and dare I say, useful.
Linking to 4chan, if you don't know what it is, is dangerous - so I won't. But the site is basically an image board, where all content is user generated, and no one needs to log in. Reflecting the permissive nature of the site, comments range from the downright obnoxious to the hilarious. There is an underlying element of mischief and theatrical excess. Then there are the memes, in all their wild and unrestrained creative glory. And yes, if you only know of 4chan through traditional media, there is an abundance of adult content, limited to a minority of the boards. Try filtering the boards to "work safe" to get some real value out of the site.
To the original point of this post. Sites like the three above, probably share a large chunk of the same users (no citation for that claim). But each site brings out a different aspect of its users. This accent of the sites, is something that is self propagated over time, but is also seeded by the site itself. A site proclaiming itself to be for the nerds, brings out the nerds. Another name woot, can never let anyone take themselves too seriously. The world wide web, is not so much a fragmentation of users, as it is a fragmentation of accents. And the tone you get in your users is, in all probability, the tone you portray in your site to begin with.