Researchers have demonstrated a new technique capable of stealing personal information from Facebook.
Using 'socialbots', computer programs that mimic real Facebook profiles, the researchers were able to harvest vast quantities of personal data.
Socialbots are increasingly being used by internet criminals and are being offered for sale on the internet for as little as $29 (£18).
Facebook said that the research was overstated and unethical.
A socialbot is a social networking adaptation of the wide-scale botnets used by criminals to send out spam.
Making friendsIn a traditional botnet, a network of computers are infected by a virus to allow a hi-tech criminal to use them remotely. Often botnet controllers steal data from victims' PCs or use the machines to send out spam or carry out other attacks.
What makes a socialbot different is that it is able to pass itself off as a real Facebook user.
The software takes over control of a social networking profile and from there performs basic activities such as posting messages and sending requests.
The four researchers from the University of British Columbia in Vancouver, created 102 socialbots for use in their experiment and one 'botmaster' - software that sent commands to the other bots.
The researchers employed their socialbots over a period of eight weeks. In total the bots attempted to make friends with 8,570 Facebook users. 3,055 accepted the friendships.
The researchers found that the more friendships people had on Facebook, the more likely they were to accept the 'fake' friend.
To prevent triggering Facebook's fraud detection software, the fake accounts only sent 25 requests per day.
PhishingFrom the profiles of those they befriended and the extended networks of those friends, the researchers claimed to have 'stolen' 46,500 email addresses and 14,500 home addresses.
In their paper, due to be presented at next month's Annual Computer Security Applications Conference in Florida, the researchers wrote: "As socialbots infiltrate a targeted online social network, they can further harvest private users' data such as email addresses, phone numbers, and other personal data that have monetary value."
"To an adversary, such data is valuable and can be used for online profiling and large-scale email spam and phishing campaigns."
Facebook said that the experiment was unrealistic because the IP addresses used came from a trusted university source, whereas the IP addresses used by real-life criminals would raise alarm bells.
It also said that it had disabled more of the fake accounts than the researchers claimed.
"We have numerous systems designed to detect fake accounts and prevent scraping of information. We are constantly updating these systems to improve their effectiveness and address new kinds of attacks," said a spokesperson.
"We use credible research as part of that process. We have serious concerns about the methodology of the research by the University of British Colombia and we will be putting these concerns to them.
"In addition, as always, we encourage people to only connect with people they actually know and report any suspicious behaviour they observe on the site."
Ethical?The researchers estimated that a real-life malicious attack could have a success rate of 80%.
"Online social network's security defences, such as the Facebook Immune System, are not effective enough in detecting or stopping a large-scale infiltration as it occurs," they concluded.
"We believe that large-scale infiltration in online social networks is only one of many future cyber threats, and defending against such threats is the first step towards maintaining a safer social web for millions of active web users."
Consultant from security firm Sophos Graham Cluley said the research was "interesting"
"Clearly there's a lesson for Facebook users to learn there about the need to carefully vet who you allow to become your Facebook friend, and what information you choose to share online,"
he said in his blog.But he questioned how ethical such research was.
"Facebook's security team is unlikely to look kindly on people who conduct experiments such as that done by the university researchers, and users are reminded that under Facebook's terms of service you are not allowed to create fake profiles, should use your real name, and should only collect information from other users with their consent," he said.
