On Monday, the Marine Corps issued an administrative directive saying it was banning the use of social networking sites such sites as Facebook, Twitter and MySpace.
The Marines, in a statement, said the "very nature of social networking sites creates a larger attack and exploitation window, exposes unnecessary information to adversaries and provides an easy conduit for information leakage."
The order doesn't affect Marines' private use of such networks on personal computers outside of their jobs.
The Marines' computer network already effectively blocks users from reaching social networks, officials said. Marine officials said part of the reason for the new ban was to set up a special waiver system that governs access for Marines who need to reach the sites as part of their duties.
The Pentagon is also taking a harder look at Web 2.0 sites. The agency is reviewing the use of Facebook and other social networking sites on its computers with an eye toward setting rules on how to protect against possible security risks.
Deputy Defense Secretary William Lynn ordered a review of both the threats and benefits of using emerging Internet capabilities, which the military has widely used for recruiting, public relations and sharing information with allies and military families, officials said Tuesday.
Lynn noted that the sites and other Web 2.0 capabilities "have rapidly emerged as integral tools in day-to-day operations across" the department.
"However, as with any Internet-based capabilities, there are implementation challenges and operations risks that must be understood and mitigated," Lynn said in a memo issued Friday.
He said he wants the report by the end of the month on the subject and that the Pentagon will issue a policy no later than Sept. 30.
The U.S Strategic Command also last month issued a warning to all services that it was thinking about a ban on Web 2.0 sites.
The various local network administrators, military services and base commanders already may have other systems for blocking certain kinds of use, but officials are trying to come up with a uniform policy for across the department, Lt.Col. Eric Butterbaug, a Pentagon spokesman, said Tuesday.