It is the responsibility of these site owners to ensure that false birthdates are not accepted, if needed to have a separate site for kids under the age of 16. Any account created on these sites have to be validated, thus limiting the posting through genuine accounts and not fake accounts. This will go a long way in preventing cyber-bullying.

During July, August, and September, the Office of the Children's eSafety Commissioner investigated 70 serious Australia-wide cyberbullying claims, up from 40 in the same period last year.

http://www.zdnet.com/article/childrens-esafety-commissioner-reports-75-percent-increase-in-cyberbullying-complaints/