Users are being warned not to post personal details on their profiles
Get Safe Online, a government-backed campaign group, is warning against posting personal details online.
Its research also showed eight million people leave home wireless networks unprotected against intruders.
And more than half of the over-65s polled use a single password for every website they visit, the group said.
The group is advising older internet users to change their passwords more often.
Tony Neate, managing director of GetSafeOnline.org, said a date of birth and address details were enough for someone to set up a credit card in another name.
"Although some of these details may seem harmless, they actually provide rich pickings for criminals," he said.
Cabinet Office minister Gillian Merron said the risks can be easily fixed and did not mean people should stop using social networking sites and wireless networks.
The poll of 2,000 adults also showed that nearly 30% admitted searching for former girlfriends and boyfriends on the sites ,and almost one in three used them to find out about their boss, colleagues or a job candidate.