Mary has a great list of ways where linkages between people (ie nodes) are generated. But, the connections between nodes are only half the battle. The important measurements of a network is the analysis that occurs beyond the interactions between two actors. In other words, you have to study 3 actors in a play to understand the relationships and value (or lack thereof) that each provides. (Shakespeare anyone?)
I read this book awhile ago, but I do remember two important factors in analyzing a social network: density of connections among nodes in a network and direction of relationships.
For example: I link to Andy. I link to Sooz. They both link to me. But, have you ever seen either one of them to link to each other? I link to John Battelle but do you ever see him link to me? Allen Searls links to me incessantly (Thanks Allen!), and I link to him only occasionally. Allen doesn't link to Sooz or Andy. Andy, Allen and I have all linked to John before. I don't think John has ever linked to any of us. We can grasp the dynamics of all of this because it is on a small scale. (eg We are all infatuated with John. He's too busy to engage us. Andy and Sooz butt heads. Allen is drinking my kool-aid.)
Q: But, what happens when the scale gets bigger?
A: (Yes, I am interviewing myself.) Of the 250 odd people that read my blog and the 300 or so that have linked to it, how many of them are actually people that have a meaningful relationship with me? Or even just a two way relationship with me? How many of them know someone I know? Or 5 people that I know? Do those people have meaningful relationships with each other?
I have no freaking clue. But, damnit! I would love to know.
I am not working on this opportunity/problem right now and don't have the time to do a more complete survey of it. But, the people are out there that know how to do the social network analysis math. They just need to hook up with the people that are doing the indexing, crawling and analysis of our RSS feeds.
As I commented over here, this is a huge commercial and social opportunity.