They're designed as clubs so that kids who want to join have to register. Registration spells protection in the sites who do this right. For example, a child has to be a member to chat in or post to discussion boards on these services, and members have gone through that registration screening process. All three are known for their sound kids' safety and privacy policies. Chat rooms also have profanity filters which delete inappropriate language.
Parents can even edit the list of addresses from which kids can receive e-mail.
They're fun places for kids that have excellent safety features.