I think this is a similar question to when kids should be given a cell-phone. The answer I think is less of when and more of how. Because both are now pretty much essential technologies, I think it is appropriate that children are introduced to them pretty early (6th-8th grade perhaps), but that doesn't mean they are given without any guidelines or rules. A phone for instance might have limited minutes and have rules that they are never used during school or after a certain time at night.
For a social networking site, I would also have some time limit rules, and I would only allow then to join a network to which I also belonged and was friends with them, and I would make sure I knew the people they were adding as friends, and make sure their privacy settings are high.