Sonia Livingstone, professor of social psychology at the London School of Economics, said the number of young children joining Facebook was rising, even among those of primary school age.
The social network asks users when they were born but it does not carry out any checks. Miss Livingstone called on the company to “do more” to protect young children.
Privacy campaigners have raised concerns about young people being bullied on the website, or being befriended by paedophiles.
“Facebook could be doing more,” said Miss Livingstone. “It starts at the beginning of secondary school as part of the Year Seven experience. They arrive at a new school, they have lost their contacts from before and that’s the moment when it starts [but I also] see evidence that it is coming down into the upper years of primary school.”