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How did he know that?” we ask. How did I lose control of who knows about my traumatic childhood, my penchant for tasteless humor, or my vacation to the Dominican Republic? You may know this feeling: you felt it when your mother friended you on Facebook, or on any other social networking site that used to be just you and your friends. Privacy violations are intrusions.
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Bruce Schneier (Data and Goliath: The Hidden Battles to Collect Your Data and Control Your World)