Researchers found that adults with Williams syndrome - who are extremely social and trusting - use Facebook and other social networking sites frequently and are especially vulnerable to online victimisation.
Around a third of participants in the study said they would send their photo to an unknown person, arrange to go to the home of a person they met online and keep online relationships from their parents.
"You have this very social group of people who are vulnerable in real life and now they are seeking a social outlet through the internet, communicating with people they know and do not know," said Marisa Fisher from Michigan State University (MSU) in the US.
Williams syndrome is a relatively rare genetic disorder characterised by developmental delays, learning disabilities, excessively social personalities and an affinity for music, researchers said.
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Many adults with the syndrome live with their parents or other caregivers, they said.
Nearly 86 per cent of adults with Williams syndrome use social networking sites such as Facebook nearly every day, typically without supervision, the study found.
Participants also share a large amount of identifiable information on their social network profiles and are likely to agree to engage in socially risky behaviours, researchers said.
"It is time to start teaching individuals with Williams syndrome about safety, both in the real world and online," said Fisher.
"This includes what personal information they should share, how to set privacy settings and how to decide whether an 'online friend' should become an 'offline friend,' she said.
The findings were published in the Journal of Intellectual Disability Research.