With social networkings being the next best thing to actually seeing your friends in person, there is a lot of information that is contained within those websites. What may surprise you is that the information you think is exclusive to that website is actually available to the entire internet, in most cases.
Take a look at all of the social networks that you belong to and count the number of times you have some sort of personal information posted on the website. Personal information includes, addresses, phone numbers, school information, email addresses and instant messaging screen names.
To go into this a little further, with an AOL Instant Messanger (AIM) screen name, it is possible for me to find out a lot of information about someone who has information posted on various social networking sites. With Facebook, I can search for people based on tons of different criteria; phone number, address, email and screen name. Those are just a few of the options one can search by with Facebook. Securing your profile makes it so people cannot view your profile, however it does not prevent people from seeing your name in the search results.
I have said this in numerous posts, but I am going to say it again. Once you put something on the internet, it is on there forever. Putting any information on the internet is like putting it up on a billboard at church. Everyone can see it, and everyone can find it. Privacy settings help a little bit, but in the end you have no control over what your friends do with your information that they can see.
It is a very scary thought to see what one person can do with with such little information.