Just saw you've got the Blog That Celebrates Itself compilation - nice one. We're on the Pink album :) Since you love Blonde Redhead, SPC ECO & M83, I trust you'll also enjoy Ummagma - You can download BOTH our albums for free at http://ummagma.bandcamp.com - Hope you'll have a listen (and thanks if you do).
What I'm taking too long to get at, is: 1) the more general the internet service the more we try to make our username mimic our given name because these kinds of services give others the broadest view of ourselves, 2) discreet or single-interest sites that require usernames follow a more fictive trend because we have already decided to offer only a small facet of our personality. The fact that these fictive usernames are always the same, even if we're showing different facets, is probably just a matter of convenience.
Most usernames are a strange phenomena. Its as if everyone in our generation were given the chance to name themselves at the age of 14. We take parts of ourselves that we think others will find interesting, and put them in 16-character made-up words. Its a horrible, horrible situation. Google usernames- and to some extent Facebook URL names-- try to approximate your meat-space name because these are meant to extend basic social interaction. Whereas Last.FM or Flickr is meant to show very particular parts of ourselves. AIM and Twitter names seem to be a paradox because they are purely social tools, but we name ourselves in ways similar to themed sites. I think it is because they offer a very discreet service. Google and Facebook are supposed to be hubs and aggregators of our lives on the web and we try to relate them to our physical bodies as closely as possible.