It's up to the individual what he or she puts on their page.
A lot of people using them are too young to realize the internet is forever! With just a few google searches a prospective employer could learn more than you ever wanted them to know about you.
There are 2 reasons why I feel social networking is popular.
1. It makes a lot of people feel like they are celebrities!
They posts photos, videos, poems, songs, and their thoughts for the world to read. The word "friend" has been so watered down. How can anyone have 2500-5000 "real friends" on Face Book? If you don't know your friend's birthday, have their home address, telephone number, or you've never been in the same room together....etc They're probably not a "real friend".
2. It's a very cheap way of staying in contact with your "real friends and family". These internet sites can also be used for business networking and more often these days as a way to meet people for the purpose of dating, sex, and even marriage.
It all comes down to how you use it.
Just remember nothing ever really disappears in cyber space!