It appears that Facebook will be heavily promoting users to download (and buy) McAfee VirusScan. It’s possible that this could be another revenue stream for Facebook (gotta keep it free somehow), but there are plenty of free anti-virus apps available.
For Windows
Microsoft Security Essentials
As much as I hate to say this, but MSE was actually rated one the best AV application available (and the only free one among the best). Small, lightweight, and fast. Available for Windows XP, Vista, and 7.
For Mac OS X
ClamXav
Although Mac OS X is more secure than Windows, it still doesn’t hurt to be cautious. ClamXav is based on the open-source ClamAV software.
For Linux-based OSes
Avast! Linux Home Edition
I’m going with Avast! over AVG mainly because AVG doesn’t provide any information about their Linux product on their web site (although it is available on their download page). Of course, you can always go with the previously mentioned open-source ClamAV as well.