If you use Facebook, I urge you to read this article, A refresher course on Facebook privacy controls. (http://windowssecrets.com/top-story/a-refresher-course-on-facebook-privacy-controls/) It is written by Patrick Marshall. He is a regular technology columnist for The Seattle Times. He has also written for Government Computer News, InfoWorld, PC World, the Congressional Quarterly, and other publications.
To quote one, hopefully eye-catching, sentence from his article:
“There’s one other frequently misunderstood fact about Facebook privacy: Because friends of your friends can see some of the things you post and tag — even if you’ve set more restrictive settings — you’re never sure just who is seeing what.“