Younger children should be supervised on a computer anyways- this site is very useful when needed to keep in contact with people or just wanting to have fun. You can also block stuff on facebook which makes for great privacy and security issues!
In my opinion, the Facebook is the main social network with a lot of possibility, application and information. Users should be careful with their personal settings (and these are a lot!).
Everyone can report and block spammers or other privacy attack, adult or illegal content, fake profiles, etc.