The biggest reason with social networking and the military is that when a service member dies, his buddies tweet, facebook,etc. before the military has enough time to contact the next of kin. You guys can see the problem here.
The other reason, is when the service member drops clues to his next mission, ie, "tomorrow is another big day, going to ______." And if the enemy wants to see what they are up against, all they have to do is look in the photos of weapon systems.