Different US military installations seem to have different standards on what social media sites are blocked and it looks like at the least the Army is trying to standardize what social media sites should be allowed:

The Army has ordered its network managers to give soldiers access to social media sites like Facebook, Flickr, and Twitter, Danger Room has learned. That move reverses a years-long trend of blocking the web 2.0 locales on military networks.

Army public affairs managers have worked hard to share the service’s stories through social sites like Flickr, Delicious and Vimeo. Links to those sites featured prominently on the Army.mil homepage. The Army carefully nurtured a Facebook group tens of thousands strong, and posted more than 4,100 photos to a Flickr account. Yet the people presumably most interested in these sites — the troops — were prevented from seeing the material. Many Army bases banned access to the social networks.

An operations order from the Army’s 93rd Signal Brigade to all domestic Directors of Information Management, or DOIMs, aims to correct that. Issued on May 18th “for official use only,” the document has not been made public until now.

It is “the intent of senior Army leaders to leverage social media as a medium to allow soldiers to ‘tell the Army story’ and to facilitate the dissemination of strategic, unclassified information,” says the order, obtained by Danger Room. Therefore, “the social media sites available from the Army homepage will be made accessible from all campus area networks. Additionally, all web-based email will be made accessible.”  [Danger Room]