By Jim Finkle - Analysis BOSTON (Reuters) - Cybercrime is rapidly spreading on Facebook as fraudsters prey on users who think the world's top social networking site is a safe haven on the Internet. Lisa Severens, a clinical trials manager from Worcester, Massachusetts, learned the hard way. A virus took control of her laptop and started sending pornographic photos to colleagues. "I was mortified about having to deal with it at work," said Severens, whose employer had to replace her computer because the malicious software could not be removed. Cybercrime, which costs U.S. companies and individuals billions of dollars a year, is spreading fast on Facebook because such scams target and exploit those naive to the dark side of social networking, security experts say. While News Corp's (NWSA.O) MySpace was the most-popular hangout for cyber criminals two years ago, experts say hackers are now entrenched on Facebook, whose membership has soared from 120 million in December to more than 200 million today. "Facebook is the social network du jour. Attackers go where the people go. Always," said Mary Landesman, a senior researcher at Web security company ScanSafe. Scammers break into accounts posing as friends of users, sending spam that directs them to websites that steal personal information and spread viruses. Hackers tend to take control of infected PCs for identity theft, spamming and other mischief. Facebook manages security from its central headquarters in Palo Alto, California, screening out much of the spam and malicious software targeting its users. That should make it a safer place to surf than the broader Internet, but criminals are relentless and some break through Facebook's considerable filter. The rise in attacks reflect Facebook's massive growth. Company spokesman Simon Axten said that as the number of users has increased, the percentage of successful attacks has stayed about the same, remaining at less than 1 percent of members over the past five years. By comparison, he said, FBI data shows that about 3 percent of U.S. households were burglarized in 2005. "Security is an arms race, and we're always updating these systems and building new ones to respond to new and evolving threats," Axten said. When criminal activity is detected on one account, the site quickly looks for similar patterns in others and either deletes bad emails or resets passwords to compromised accounts, he said. Facebook is hiring a fraud investigator and a fraud analyst, according to the careers section of its website. CANNOT GUARANTEE WEB SAFETY But ultimately Facebook says its members are responsible for their own security. "We do our best to keep Facebook safe, but we cannot guarantee it," Facebook says in a warning in a section of the site on the terms and conditions of use, which members may not bother to read. (Statement of Rights and Responsibilities | Facebook) Continued... "People implicitly trust social networking sites because they don't understand the real threats and dangers. It's like walking down the street and trusting everybody you meet," said Randy Abrams, a researcher with security software maker ESET. Amy Benoit, a human resources manager in Oceanside, California, said she may stop using Facebook altogether after she became entangled in a popular scam: A fraudster sent instant messages to a friend saying that Benoit had been attacked in London and needed $600 to get home. Yale University last week warned its business school students to be careful when using Facebook after several of them turned in infected laptops. One of the most insidious threats is Koobface, a virus that takes over PCs when users click on links in spam messages. The virus turned up on MySpace about a year ago, but its unknown authors now focus on spreading it through Facebook, which is struggling to wipe it out. "Machines that are compromised are at the whim of the attacker," said McAfee Inc (MFE.N) researcher Craig Schmugar. McAfee, the world's No. 2 security software maker, says Koobface variants almost quadrupled last month to 4,000. "Because Facebook is a closed system, we have a tremendous advantage over e-mail. Once we detect a spam message, we can delete that message in all inboxes across the site," said Schmugar. Facebook's Axten said the site does not know how many users have been infected by Koobface. A new website that follows Facebook news, FBHive « Abuzz with Facebook news, information, rumors and speculation, recently identified a vulnerability that made it possible to access any user's private information using a simple hack. The loophole has since been closed. "We don't have any evidence to suggest that it was ever exploited for malicious purposes," Axten said. Hackers even find ways to get into accounts of savvy users like Sandeep Junnarkar, a journalism professor at City University of New York and former tech reporter. Last month he learned his account was hacked as he waited for a flight for Paris. He quickly changed his password before boarding. "Am I surprised that it happened? Not really," he said.