Facebook is more or less the king of the world when it comes to social networking. It has exploded outwards which such speed as it allowed users to post their thoughts, images, join fan clubs, find long lost friends and create many a school reunion.
Intially, it was setup to allow only your friends view the content you had on Facebook. So you were safe in the knowledge that only people who you have accepted as friends on Facebook could view what you were doing.
All the different things you can do on Facebook like