They say that from just a handful of personal details posted online, it is possible to get enough information about someone to obtain loans and open credit card accounts.
Some users are also, inadvertently, inviting burglars to their homes by posting their addresses and details about when they are on holiday.
Tom Ilube, chief executive at Garlik, said: "Social networking sites invite you to give personal information that you wouldn't do if you met a stranger on the street. For some reason we are more willing to give away our information online. Fraudsters can very quickly piece together enough about you to allow them to impersonate you."
Tony Neate, a former police officer in the National Hi Tech Crime Unit and managing director for Get Safe Online, an internet campaign group, said: "Organised criminals are harvesting information about people from these sites. In a couple of hours it is possible to find out enough to completely take over someone's identity."
Employers have also begun tapping into them as a way of checking up on potential employees.