Well with what you describe, the graph would become the commodity, the sites themselves would remain unique and competitive, but as interfaces to the graph.

Overall I think your idea sounds like a win for all. Users could have a single presence at the site of their choice, with friends from other sites, new entrants would have a lower barrier of entry, and existing sites are less likely to lose users to the network effect. Of course, the big players would probably have more competition from niche players too, so I guess that would be the downside for Myspace and Facebook.