Age dependent. Kids don't need to be on these sites, & if kids do go on- only with supervision and on age-appropriate sites. Sites that allow kids to interact with careful guarding of personal information (see http://bit.Ly/c53qkq) can help kids learn limits on line. It is a fact of life that they will enter onto the internet, supervised learning with safe sites can help them adjust to others as they get older.
No. Children and even adults don't know the implications of posting information on the internet. Information posted on social networks might leave the child vulnerable to predators or problems later on in life. Parents need to closely monitor their children's internet use.
No. No, never! there is simply zero need and far too much downside.