Information that you share publicly on social networking sites like Facebook, Twitter, Google+, etc. can sometimes be used against you (such as embarrassing photos, what you think of someone, etc.). The general rule of thumb is "if its too personal don't post it", but its also good to make sure that only the people you want to see your posts can.
Social networking sites in the past have made it difficult to make your information private, but over the last few years they have gotten better about allowing you to lock down your data. Although, the problem is that these settings are not always that easy to understand what they do.
There is a new extension that I heard about recently called "Privacyfix", its supposed to help you lock down your privacy setting on Social networking sites like Facebook, Google, and others. So far I have used it to lock down my settings on Facebook, Google, but I have not used it on other sites except to block cookies.
The company claims, "We don't collect any data from your use of Privacyfix, unless you choose to send it to us. We don't store IP addresses and we cannot and do not see or save your web browsing."