facebook users under 13

in this post on the freakonomics website by matthew philips he cites a survey (social networking, age and privacy) by the london school of economics 

apparently one in every three children in the U.K. between 9 and 12 has his/her own facebook page despite the minimum age for facebook being 13.  other results of the survey are

- a quarter of 9-16 year-olds on social networking sites across europe have their profile set to ‘public’;

- 1/5th of children whose profile is public display their address and/or phone number