Computer viruses are typically designed to target popular programs ... and as such, there is now one targeting Facebook users. This virus was first discovered in August 2008 and is known by its technical name, "W32.Koobface.A" (and W32.Koob.face.worm, and Boface.A).
The best way to protect yourself against this worm is to make sure your antivirus software is up to date.
If you don't have an Antivirus program on your computer, I highly recommend Norton Internet Security (not Norton 360).
For more information about other viruses found on facebook, click here.