Sounds interesting in theory, though in practice it depends on how many people have signed up. When I played around with Friendster, my network was very sparse, with few people being bothered to sign up and hassle their friends into doing so. It wasn't that nobody would be my friend. I had plenty of those; just that very few of them bothered to actually add any other friends (or fill in more than the bare minimum of details about themselves). Mind you, the fact that Friendster is all but useless except for finding sexual partners sort of limits its audience; perhaps in Australia, using computers for getting laid is still seen as the last refuge of sad losers, rather than the playground of cutting-edge cyber-nerverts.
tribe.net would be somewhat more interesting to graph, as it's a multi-use system (useful for posting announcements, selling stuff, shooting the bull about your favourite TV show/band/sports team and meeting people), thus giving denser social networks. LiveJournal would probably yield even more densely interconnected networks; also, since LJ has an open API, it shouldn't be too hard to rig up something to spider it and plot a graph, possibly even correlated with a physical map of the real world. I wonder whether anyone has thought of this already.