Brian Solis on Huberman et al: the meaningful social network in twitter isn't the network of followers, it's replies. Followers are often very light weight ties or spammers. Replies are much more reflective of relationships.
Ross raises the issue of how to keep @replies from turning into all spam? I think "following" could be used as a whitelist. This would work, but would cut down the serendipity of asymmetrical following. If you're twitter-famous and follow fewer people than follow you, you can discover followers by seeing who @replies you. White listing removes the serendipity.
But - Twitter would be a candidate for an even better sort of spam cleaning tool. Not only could traditional spam-filtering techniques be used, Twitter could use a social whitelist. Maybe you don't follow someone but your friends do, and they're white listed too.
Maybe colors show tribal gradient. Brightest is your followers, next is your friends followers, and so on. How many degrees of separation would be computationally tractable and socially valuable?