According to security firm Sophos, Scammers are posting fake links on FaceBook accounts that, when you visit the linked site, installs the MacDefender malware onto your computer.
MacDefender is a fake security program: while promising to clean your computer of malware, it actually is malware itself, and can cause all manner of problems for you.
The supposed FaceBook links offer videos of former International Monetary Fund Managing Director Dominique Strauss-Kahn and a hotel maid, or an X-rated video of celebrities Rihanna and Hayden Panettiere.
Not that I have a clue as to who they might be.
But the upshot of this is that the videos do not exist, these links are bad, and you should just avoid clicking on these sorts of things.
Simple, really.