On social networking sites such as Facebook you can change your settings; As on Facebook there maybe people on there trying to find out your information about people. As these people are clever as some of these people may scam you. To stop this you must go to your security settings on facebook. You can change the settings so that just the people that you are Friends with on facebook can look at your settings and pictures.
When you post something on another person's wall and your settings is private but there settings isn't private and there settings are public everyone can see there post as they can re-post your post will become available to the public.