Some 7.5 million of the 20 million minors who used Facebook in the past year were younger than 13, and a million of them were bullied, harassed or threatened on the site, says a study released Tuesday.

Even more troubling, more than five million Facebook users were 10 years old or younger, and they were allowed to use Facebook largely without parental supervision leaving them vulnerable to threats ranging from malware to sexual predators, the State of the Net survey by Consumer Reports found.