Many web sites prohibit children under the age of 13. It's not necessary out of altruism, but because the Children's Online Privacy Protection Act (COPPA) contains various provisions and requirements concerning special treatment that must be accorded to the children.

The statute is designed to protect young children from web sites that might seek to exert influence over them by obtaining personal data and marketing to them. Indeed, web sites are required to get verifiable parental consent before allowing such children to participate and they must follow certain rules to safeguard their privacy.

All good, right? Not so fast.

Facebook, for example, has barred about 800,000 prospective users who are 13 or younger via a layered screening process, according to a recent Consumers Report study and a Reuters article. That is good.

However, those same sources indicate that 3.5% Facebook's users in the United States are underage children who should be banned from the site according to the site's policy against children 13 and younger. That percentage may not sound too high at first blush, but in terms of raw numbers, it translates to 5.6 million children.

Facebook globally has close to 1 billion users, so even a small percentage of the total is tantamount to a large number of real people.

Apparently, it is not too difficult for children to navigate around Facebook's screening process, which is designed to block children under the age of 13.

And interestingly, a number of parents actually help their young children to get onto Facebook, in violation of the company's policy against hosting such users.

Lawmakers and companies are trying to protect children online. But it is, obviously, not an easy task.