The Marines have banned Twitter, Facebook and MySpace, subject to exceptions for "mission-critical need." Staying in touch with family back home while fighting wars in Iraq and Afghanistan, is not mission critical.
A U.S. Strategic Command source tells Wired.com's Noah Shachtman that social networking sites made it too easy to spread "scams, worms, and Trojans" among the Marines. So the U.S. is now in the peculiar situation of deeming social networks safe enough for its teenaged girls, but too perilous for Marines, in active war zones. The Marines will remain completely safe from movie quizzes, virtual jello shot requests and online snowball wars for one year.