A while back, I posted about some "safety online" videos that Quentin D'Souza had posted. One of them shows a photo of a girl lying on a bed; the photo is posted on a bulletin board, and every time someone pulls the photo off the board, it magically reappears there. The moral: once you post a photo of yourself online, it stays there for pretty much eternity.
Today, I came across the case of a naturalized German historian who has trouble travelling freely, due in part to malicious defamations posted on his Wikipedia biography page and on Amazon.com as "reviews" of some of his books about a contentious period in Turkish history. Food for thought.