The research was carried out by Ipsos MORI for the Press Complaints Commission and involved interviews with 1,000 British web users aged 16–64.
The PCC also reported that 42% of web users aged 16–24 know someone who has been embarrassed by information uploaded on to the internet without their consent.
Social networking sites like Facebook, MySpace and Bebo are used by 83% of 16–24 year-olds who go online and half the total population of adult web users, according to the PCC. Yet only just over half of users (55%) think before posting information that it might later be used by third parties without their consent.
Public concern is demonstrated by the fact that 89% of web users think there should be clear guidelines about the type of personal information that can be published online so that they can complain if this material is wrong or intrusive, it said.
PCC Chairman Sir Christopher Meyer said social networking marks a huge cultural change in the way in which people communicate.
"Personal information is being put into the public domain on an unprecedented scale," he said. "There is a need for public awareness about what can happen to information once it is voluntarily put into the public domain."
Sir Christopher said this has implications for the PCC, which has always had the task of deciding where to draw the boundaries between what newspapers and magazines may legitimately publish and what can rightly be considered private.
"The challenge remains the same for online editorial content, including material taken from social networking sites," he said. "In the digital age, self-regulation, with its sound principles and speed of operation, has never been more relevant."
Sir Christopher said the PCC's current Code of Practice would be able to handle complaints about the use by mainstream media of information from social networking sites. He said that in the process, the Commission would "define through its decisions the boundary between the private and the public."