Infection is pretty easy you watch the video , it has an embedded malware attached to it and it gets downloaded and executed on your PC. On the other hand how its spreading is that when you click on the link it takes you to a rogue application and then the application gets the list of your facebook friends and starts posting the video link on the wall of your friends.
These kinds of events are paradise for Scammers , Spammers , Crackers etc. The best thing to protect ones self is to either use a very good AV in combination with a great antiMalware software and a personal firewall.
Again I would recommend not to click on any link related to OSAMA’s Death.