As said before, Facebook will never be shut down unless they blatantly violate international rules of conduct on the Internet such as using the site to illegally traffic humans, drugs, weapons and other contraband as well as carrying out communication with known terrorists groups to plot terrorist actions against governments, leaders, dignitaries, ambassadors and such to internationally recognized governments. So far Facebook has not done that. Facebook abides by all of these laws as well at US Federal laws. They remove users who violate these laws as well as those who use Facebook to violate these laws.
Again Facebook will not shut down unless it becomes absolutely defunct such as the way mIRC, Web Crawler, MSN Chat, and others have gone. MySpace is currently dying.