Interesting, I wonder what a sceintific study of the overlaps would actually show.  I would have said the similarity was stronger between users of reddit and Twitter.  I see the same topics discussed with the same basic mix of opinions and generally the same underlying sources.  It seems I generally end up on 8chan when drama happens and gets referred to on Twitter (meaning there is a fair bit of overlap of Twitter and the chans, too, I guess).  It is hard to remove oneself from the equation, I think, so we overgeneralize based on our own experience.