In the U.S., a law called COPPA creates additional requirements for websites that interact with children under the age of 13. So, to avoid those complications, Facebook sets their ToS to 13 and up, like a lot of other sites.
Thus it seems unlikely that they can admit that there are a lot of under-13 kids on their service, without creating the chance that some enterprising fed will take a look at their COPPA compliance. This will hopefully have the side effect of reducing the value of "for the kids!!" arguments.
