UPDATE: NextGov says this and similar reports were in error and the Marines did NOT ban Twitter. See: Marines did NOT ban Twitter!
Marines Ban Twitter, MySpace, Facebook, by Noah Shachtman, Wired Danger Room (August 3, 2009).
The article also reports that, “The Chairman of the Joint Chiefs of Staff has 4,000 followers on Twitter. The Department of Defense is getting ready to unveil a new home page, packed with social media tools. The Army recently ordered all U.S. bases to provide access to Facebook. Top generals now blog from the battlefield.”