Social Media (Facebook, Twitter,Flickr, Google+, Habbo, WeeWorld, etc. - internet sites that allow individuals to post material to share with their friends (and often others)) is a topic worthy of separate discussion.
One quick comment is that the starting point for ones thoughts on this is that you should not put anything out on the network that you are not ready for the entire world to see. And to remember that once something is 'out there' you should assume that it is there forever.
A good video on the topic is: (on youtube - not visible at school without a teacher's override)
http://www.youtube.com/watch?v=Esj-PBmXjCU&safety_mode=true&persist_safety_mode=1&safe=active