There are 7.5m Facebook users under the age of 13 in the US

"A survey published in the June issue of Consumer Reports found that "of the 20 million minors who actively use Facebook," 7.5 million were younger than 13 and more than five million were younger than 10. Facebook's terms of service require that users be at least 13.
The report tracks with other studies including a 2010 study by McAfee that found 37 percent of 10 to 12 year olds are on Facebook and a study (PDF) released in April from the London School of Economics EU Kids Online project that found that 38 percent of 9- to 12-year-old European children used social-networking sites, with one in five using Facebook, "rising to over 4 in 10 in some countries."
Jeff Fox, technology editor for Consumer Reports, said it was troubling that the survey found "a majority of parents of kids 10 and under seemed largely unconcerned by their children's use of the site." In a press statement, Consumer Reports said that "Using Facebook presents children and their friends and family with safety, security and privacy risks" and that, "In the past year, the use of Facebook has exposed more than five million online U.S. households to some type of abuse including virus infections, identity theft, and--for a million children--bullying.""

Source:  Research by Consumer Reports, reported by CNET, 10th May 2011

Note - while it doesn't explicitly say so, I'm assuming that this is US data only.  With over 600m global users, I'd expect far more than 7.5m to be under 13