Ana
hits on something that's been bothering me for awhile: All these graphs about social media influence that show a bunch of concentric circles are wrong. She wrote: "Aside of the fact that this kind of thinking is oversimplified and wrong for the obvious reason (a person who's a flickr follower is also a twitter follower is also a facebook friend, so these circles are hardly ever concentric), there is a more important one. Interpersonal influence is not broadcast. Simply put: how influential you are going to be does not only depend on you, it depends on how influential your friends are. That is, your influence is the outcome of their network even more so than yours. That applies to plain scale (how many people are in their network), but also on scope (who they are). So, hard as it may be on the ego, you are not at the center." (I added in the comments that the subject matter of the communication had a lot to do with influence as well.)
I wrote a bit about this
back in 2006, specifically pointing the confusion in these types of graphs between reach and influence.