Robert Shippey asked me a simple but intriguing question: What does a 'good' social network on Twitter look like? What makes it so tough is that it depends on what your goals are. Here's a couple of different answers:
You need a network that's very tightly interwoven with thick links, where most of your contacts talk to lots of other people you know. The advantage of this structure is that it promotes trust, since if you betray anyone in the group they can retaliate by trashing your reputation with all your mutual friends. The deeper level of trust makes it a very safe environment to build friendships and collaborate.
That high cost for any transgressions has a flip side though. It's like a small town, the same mechanism also punishes non-conformity so it's hard to innovate or do anything controversial.
The most influential people have networks that connect lots of different groups of people. That means they tend to look like a wheel; the influencer at the center with links radiating out to people who are otherwise unrelated. The advantage of diversity is that you can act as the bridge between unconnected communities, so when a good idea appears in one area it can be transmitted to others where it could be useful.
Ronald Burt uses a closure metric to measure how tight-knit your network is. In simple terms, it's the chance that for any random person in your network, they'll know a second random person from your contacts. If everybody in your circle talks to everyone else, then that probability will be 100%, but if you're only connecting people who otherwise don't know each other it will be zero. I might add that to the profiles to give people an idea of where they fall on that spectrum.
Pete Warden's blog 