Even if I take your bait and leap to the conclusion that everybody spends their life pruning their Facebook friend lists, have your friends never said anything embarassing about you? As a prank, in a fight, etc.?
Thanks to Facebook, it can be in a thousand people's news feeds in a minute, and with the permanence of computers and the internet, saved forever on a hundred hard drives, some of which belong to people who are definitely not among your friends (that includes Facebook the company itself).
