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. A great source for finding out what filtering software has been hacked or disabled is Peacefire.org , Youth Alliance Against Internet Censorship. This group of young activists offers instructions in its site on how to disable a number of filtering products. The breakdown on youth is 22.5 million kids and teens online right now, growing to 42.1 million by 2003.