Hmmm... my FB friends seem to fall into six categories similar to Salmissra's: friends from friends from grad school (biggest group), childhood/middle/prep school (second biggest group), friends from college, people I've met randomly in real life, co-workers/former co-workers, and a very small number of relatives. The first four groups have a great deal of overlap.. for some reason I tend to bump into the same people over and over again in life....

I have never defriended anyone. I've been defriended twice.. both by childhood acquaintances I long-since lost touch with, whom I suspect just wanted to check out what I've been up to.

FB used to have some good network visualization tools for friends, so that you can graphically see how your network breaks down, but most of them have stopped working.