Facebook on Tuesday said that it had partnered with McAfee to offer its 350 million members a complimentary six-month subscription to McAfee security software.

"Now, if your computer is infected, you will be asked to run a scan ... and clean it before accessing Facebook," said Facebook project manager Jake Brill in a blog post. "We're not aware of another free Internet service that takes this much responsibility for helping people keep their accounts secure."

After the six month period, Facebook users who chose to renew the subscription will have to pay a fee. A McAfee spokesperson said the fee had not yet been determined and that it will probably be less than the annual subscription price charged to retail buyers of its security software.

McAfee's software requires a PC. A Facebook spokesperson was not immediately able to provide data about the number of subscribers who use Mac OS X or Linux.

According to NetApplications, about 5% of Internet users are running Mac OS X and about 1% are running Linux. That suggests about 21 million Facebook users aren't helped by this partnership, though their computers may face less risk from malware.

Facebook and other social networking sites have become targets for cybercriminals because of their popularity, the potential value of friend data to fuel social engineering attacks, and the tendency of users to trust messages that appear to come from friends.

In 2009, Facebook had repeated problems with the Koobface worm.

Even if no other free Internet service takes as much responsibility to keep user accounts secure, as the company claims, Facebook may need to do still more.

On Wednesday, security researcher Ronen Zilberman published information about what he claims is a new type of vulnerability present in several major social networks, including Facebook.

The vulnerability, which he calls Cross Site Identification, appears to be similar to Cross Site Request Forgery. Using this technique, an attacker who manages to lure a victim to a Web page with embedded malicious content can steal Facebook data if the victim is logged in to Facebook in a different Web browser tab.