A large percentage of the population uses social media sites such as Facebook as a place for communication and sharing (often oversharing) the details of their personal lives. Facebook and other such sites gather an incredible amount of information about you, and their policies change from time to time. Knowing what settings you need to check to keep things as private as possible can be complicated. We may think we have the privacy settings figured out and know who is viewing our posts and pictures, but one policy update or added feature and suddenly activity you meant to be private could be public, or more public than you had hoped. You certainly can't count on anything you post on your wall or anyone else's to remain just between you and a handful of people.
Even if you are on top of all the settings, your friends and even your apps can share information about you that might unintentionally incriminate you. Services like Facebook Places and apps like FourSquare and Google Latitude can give away your physical comings and goings. Friends might check you into places, tag photos of you, or make offhand comments online that could expose your lies. So even more likely than someone purposefully recording your every word or move is accidental betrayal by social media.
There was a recent, albeit innocuous, incident with Facebook founder Mark Zuckerberg's sister Randi, where she posted a photo for friends only, but because of the way Facebook photo tagging works, a friend of another sibling saw and shared the image via Twitter. While the photo was a relatively benign image of the family standing in a kitchen sending smartphone messages to each other, Randi expressed her objection to the unauthorized share and the Twitter post was removed. But the image lives on via various articles. This had nothing to do with cheating, but it just goes to show you that no one is immune to accidental public sharing. Anything you or your friends do or say or post on a social media site could easily make its way beyond its intended audience, say to your spouse, who thought you were at work Tuesday night when you were really chugging a beer with your fling at a party.