Millions of people are leaving themselves open to identity theft when using social networking websites, according to the consumer group Which?
Personal information is stored on 700 databases
Members of sites such as Facebook can join large networks which reveal personal information to thousands of others on the network.
Which? says people are at a greater risk of being targeted by fraudsters than they think.
On average, UK residents' details are held on about 700 databases.
Which? says that fraudsters can use the internet to gather personal information which could then be used to trick people into revealing Pin numbers and other security information.
These could then be used by conmen to apply for credit cards or loans in somebody else's name.
Burglars could also benefit from such information, it says.
Risk from "friends"
Personal profiles that may include a date of birth, contact details or links to family and friends can often be seen by thousands of people in a Facebook network if users stick with the default privacy setting.
PERSONAL DATA TIPS
Regularly check your credit file
Check bank statements thoroughly
Immediately cancel lost or stolen cards
Shred unwanted documents
Never share bank details following an unexpected request
Install up-to-date security software
Understand privacy settings on social networking websites
"Facebook encourages users to customise their security settings to a level they are comfortable with," a Facebook spokesman told Which?
The Information Commissioner's Office is investigating why personal information remains on Facebook's computers after users have deactivated their accounts.
Awareness of identity fraud has increased since the high-profile loss of government disks containing personal data.
But, unlike government departments and public bodies, private companies are under no obligation to admit to security breaches.
To test how easy it would be to find personal data about somebody on publicly accessible websites, Which? asked a researcher to investigate editor Neil Fowler.
Using only Mr Fowler's name and occupation, the researcher was able to find out details including the names of close family members and the floorplans to his home, including access points.
"It was a real shock to see how much personal information about me could be found online, which could potentially be used by crooks to commit fraud," said Mr Fowler.
"We all need to take steps to protect our data - both online and offline - by being more aware of how our personal data could be used and taking care who we share it with."