My primary objection is the increasing incidence of cyberbullying. In Britain, for example, one in three children are victims of such crimes Social media is seen by many young children as an outlet to be rebellious and cool -- and an area where they feel a false sense of safety while talking to absolute strangers. Young children do not have the knowledge and the experience to weed out the dangerous. Let's give kids a chance to remain kids a little longer.
You can't stop progress. Kids under 13, like it or not, are digital natives in ways we will never know or understand. And they are growing up with social media. Banning them will not stop them from joining, nor will it reduce incidents of cyber bullying or stalking. What Facebook should do, is let them join legally, and place special restrictions or extra security on accounts owned by kids under 13.