Back in April, Facebook launched the Facebook Anti-Virus Marketplace, which allows users to obtain free trials of anti-virus software from big names like McAfee, Sophos, Symantec, and Microsoft. They also incorporated those company's databases into their own URL blacklist database to better help them sniff out potential threats on the site.
Since then, Facebook has provided free anti-virus products to users - but only once they have identified a threat and notified the user in question.
Now, in order to allow users to be more "proactive," Facebook has set up a self-enrollment malware checkpoint that give users access to either McAfee's Scan and Repair or Microsoft's Security Essentials.
From a note on the Facebook Security page:
Previously, if you suspected you may have malware installed on your device, you would either need to run anti-virus on your device or wait until Facebook identified an actionable threat. Now, with our new self-enrollment malware checkpoint, you will be able to proactively obtain your choice of a free anti-virus product to scan and clean your system.
We currently offer two products for all of our users: McAfee's Scan and Repair and Microsoft's Security Essentials.
The McAfee option will download a small program onto your Windows computer to perform a one-time scan of your system for malware. It will not interfere with your existing anti-virus or other security products. After it scans your system, it will give you the option to automatically or manually remove the files it flags as malicious. The Microsoft Security Essentials option is a full anti-virus product. Upon download and install, it will add anti-virus software to your computer that will continue to protect your system with the latest anti-virus signatures from Microsoft.