The watchdog's annual Children's Media Literacy Audit revealed that of those under 13 on a social networking site, 83 percent say their profile can only be seen by their friends, while four percent claim their profiles can't be seen by anyone.
Furthermore, 93 percent of the parents whose children are social networking despite being under the sites recommended age say they monitor what their kids get up to.
However, 17 percent of parents with children aged eight to 12, revealed they did not know if their child visits social networking sites or not.
Ofcom also cited Nielsen internet audience research that revealed 37 percent of five to seven year olds visited Facebook in October 2009, although they did not necessarily have a profile on the site.
The Children's Media Literacy Audit revealed that downloading or watching TV and films on the web is on the up among children aged between eight and 15, while 44 percent of 12 to 15 year olds believe sharing digital copies of films and music on the web for free should not be classed as illegal.
One in five eight to 11 year olds and 48 percent of 12 to 15 year olds use blog or sites such as Wikipedia that can be altered by web users.
Nearly half (48 percent) of 12 to 15 years olds and 70 percent of eight to 11 year olds said they believed the information on these sites, while 27 percent of 12 to 15 years olds admitted they thought search engines only returned links containing truthful information.