In the following TED talk, Jennifer Golbeck outlines how your privacy is already gone. She tells the story about how the company, Target, knew a 15-year-old girl was pregnant two weeks before she told her parents she was pregnant.
Ms. Golbeck mentions a study in her talk, and the study is incredible. Read Private traits and attributes are predictable from digital records of human behavior by Michal Kosinski, David Stillwell, and Thore Graepel; PNAS 2013 110 (15) 5802-5805; published ahead of print March 11, 2013, doi:10.1073/pnas.1218772110.
By using public information, such as your Facebook “Likes”, companies and governments can accurately predict your gender, sexual orientation, race, religion, intelligence, personality traits, and even whether your parents were still together when you were 21-years-old. Imagine what the NSA can do with all of the non-public information it has about you.
Your information, however, does not reveal information about only you. The study states,
Furthermore, location within a friendship network at Facebook was shown to be predictive of sexual orientation. This speaks to the issue that information about you is easily used to understand information about your friends and family. When you do not protect your privacy, you are sacrificing the privacy of all of your friends and family.