One in four users of social networking sites unwittingly leave themselves open to crime by revealing personal details, it was claimed today.
A poll marking the launch of an internet safety campaign also disclosed what many addicts of sites such as Facebook, MySpace and Friends Reunited already suspected - up to 39% of people use them to look up old flames.
Government-backed research for the Get Safe Online week found 25% of the 10.8 million Britons registered with networking websites expose information such as contact details or dates of birth on their online "profiles".
Among 18 to 24-year-olds the proportion putting themselves at risk of identity fraud rises to 34%.
The poll of 2,000 adults conducted by ICM found 29% admitted searching for former boyfriends or girlfriends, with the 25 to 34 age group most likely to hunt out their exes, at 39%.
Nearly one in three people has also used the sites to find out about their boss, colleagues or a job candidate.
The survey showed 13% of social networkers had posted information or photos about other people without their consent, rising to 27% of 18 to 24s.
Cabinet Office minister Gillian Merron said: "The risks we are highlighting today can be easily fixed and do not mean that people should stop using social networking sites and wireless networks.
"People simply need to take a few basic steps and simple precautions which they can find by going to www.getsafeonline.org to help keep themselves, their families and their businesses safe online."
Tony Neate, managing director of GetSafeOnline.org, said: "Although some of these details may seem harmless, they actually provide rich pickings for criminals.
"Your date of birth and where you live is enough for someone to set up a credit card in your name.
"So whilst most people wouldn't give this information to a stranger in real life, they will happily post it online where people they don't know can see it."
Wireless networks also provide opportunities for criminals.
Users of home "wi-fi" who leave their systems unsecured - estimated to number about 7.8 million people - could allow criminals to hijack their computer, adapt its programs and steal information held on it, warned the campaign, which is a joint initiative between the Government, the Serious Organised Crime Agency and industry sponsors.
The poll also found 15% of people do not use any privacy settings on social networking sites.
And almost one in four (24%) people use the same password for all websites.Reuse content