Why are 5 million kids on Facebook if it doesn't want them?

Facebook has an ugly little secret, a number disclosed nowhere in its voluminous filings to become a public company and now only vaguely addressed by corporate officials.

An estimated 5.6 million Facebook clients - about 3.5 percent of its U.S. users - are children who the company says are banned from the site. Facebook and many other web sites bar people under age 13 because the Children's Online Privacy Protection Act (COPPA) requires web sites to give special treatment to children 12 or younger. The law aims to stop marketers prying personal information from children or using their data to advertise to them. Sites must get parental permission before allowing children to enter, and must take steps to protect privacy. Facebook declines to acknowledge that many of its efforts to block children are not working.