Privacy & Appropriateness
You hear in the news about people getting fired or passed up for new jobs because of Facebook posts. Micheal Ryan made this great chart to help people understand what degrees of privacy different communications have.
What makes these sites confusing for people is that you can be a very different person with family, friends, classmates, associates and co-workers. Before, interaction with these different groups were kept separate and now they are all mixed up together. I tell people to act as if there Mom and their boss are going to read all their posts and see all their pictures. (My mom is on Facebook, so this scenario is not too far fetched.) How would your boss react if you complained about a customer? Or you called in sick and there are pictures of you at a party the night before?
Remember that YOU are responsible for what you post.