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Defence department officials say the benefits of using social media now outweigh the risks to security.The ruling means that a number of sites blocked by the Pentagon in 2007 - including YouTube - will be unblocked.
However, commanders will be able to temporarily block access to safeguard missions or save bandwidth.
Correspondents say social media and the internet are becoming increasingly important for the US military.
The web is used for recruitment, public relations and for communications between troops and their families, among other things.