If you are active Facebook user and you like data visualisation just like me, then I strongly recommend you look at Social Graph application. When you run in, a Flash applet draws circles representing all your friends and connections between them. You are not there because, well, these are your friends so you are connected to all of them. You can only see how are they connected to each other. What's most interesting though is that they form some groups, which are clearly depicted thanks to graph visualisation algorithm used in this app. For example, my graph looks like this:
Several persons are not connected to anyone other I know. They are drawn somewhere outside the graph in a far distance. These are people who I know from some unusual place or who do not participate actively in Facebook and do not collect friends.
Rest of them forms clearly distinctive groups. The group on the right are people from my home town - mostly my friends from high school or university. The biggest group in the middle are passionate game developers from warsztat.gd community. Small group at the bottom consisting of four circles are contacts from my current job. On the left of Warsztat there is a group of professional game developers from Polish game industry. And finally the smaller group on the left consists of people I know from psytrance parties :)
What's also interesting is that there are some people who play role of a "center" of some group - are connected with almost everyone else in that group. Others are "connectors" that join two groups.