The worm sends you link that appears to come from a friend. Trend Micro (http://blog.trendmicro.com/new-variant-of-koobface-worm-spreading-on-facebook) reports that it looks like a YouTube video and requests that you upgrade your Flash player, but it in fact installs itself on your PC where it has access to your Facebook account and propagates itself to everyone in your friends list.
Get the latest virus definitions and be careful what links you click on to protect yourself from other worms in the future. If you don’t have anti virus on your PC check out Clam Antivirus which is open source software that works just as well as Norton or Mcafee.