Mashable has a nice list of Social Networks that are fun and safe for kids.
Children are wanting to get online and interact earlier and earlier, and here’s a list of the recommended sites:
Most of the sites offer an environment allowing the kids to interact with their peers, upload photos, play games, set their preferences (likes, etc.). All offer parental controls as well as strict sign up rules. There are also controls on who your child an interact with, links outside of the site, and other safety controls.
All the sites are great environments for kids – but also for parents who are concerned about what their children might be viewing – and who they are interacting with – online.