Fundamentalists, terrorists, call them what you like would have their legs cut away from under them if we close down these unnecessary sites now. Social networking is one of the most important tools to these illegal groups just as it is to anybody involved in criminal activity. This will be one of many measures that would help defeat terrorism.
No, social networking sites should not be shut down in times of emergency, because that is when they are most important. In times of distress, social networking lets people know what areas need help. They allow people to alert their family and friends that they are safe. Shutting them down would only be a way to control the world.
In case of an emergency, people should know, and the best way to spread the word is social networking, which is why anyone would want to shut it down. Think of what happened on 9/11, in the world trade centers, people were told to stay calm and they would be taken out of the building. Maybe if they had ran like hell many more would have survived, but instead, they did as they were told and lost their lives.