I don't think I can agree that race or ethnicity determines where people choose to participate online. I think that for some people that may be the case (and even those will tend to participate in some that are not race or ethnicity specific), but I would suggest that most internet fora/message boards, etc, are subject related, and that the majority of subjects are not conducive to drawing people of any particular race, or ethnic group.

While some interactive websites are political in nature, the vast majority are not. They're about things like diabetes, weight loss, car repair, gardening, favorite tv shows, favorite musicians, favorite movies, favorite recipes, and they are social networking sites like facebook, myspace, twitter, and things like that. These kinds of websites and webservices don't tend to encourage forming social groups based on race or ethnicity.