Facebook today announced the Antivirus Marketplace, a partnership with Microsoft, McAfee, Trend Micro, Sophos, and Symantec, to provide the social network’s users with free six-month licenses to anti-virus software.

The anti-virus partners will augment Facebook’s URL blacklist system with their own URL blacklist databases. The companies will also contribute to the Facebook Security Blog to provide information about how users can protect their accounts and keep Facebook safe for other users.

With 901 million monthly active users, the social network is a prime target fo malware, viruses, phishing attacks and spam. Facebook has made several improvements to its automated enforcement — including self-XSS protection and quickly blocking apps, profiles or links that a certain percentage of people mark as spam — and user-facing security protections, like login approvals and remote log-out, but providing anti-virus software adds another layer of security.

The company has worked with McAfee in the past to give users free and discounted anti-virus protection. Symantec, which produces Norton anti-virus software, has maintained an app on Facebook since 2010 that scans users’ News Feeds for unsafe links. The company also helped Facebook identify a data leak in 2011. Sophos is an interesting partner since the security software developer has been quite critical of Facebook in the past.

Users can visit the Facebook Security page to access the free software downloads and receive updates about protecting their accounts.