In a very thoughtful and well produced document the United States Marine Corps outlines its position on the use of social media by its members.
The Marine Corps encourages Marines to explore and engage in social media communities at a level they feel comfortable with. The best advice is to approach online communication in the same way we communicate in person — by using sound judgment and common sense, adhering to the Marine Corps’ core values of honor, courage and commitment, following established policy, and abiding by the Uniform Code of Military Justice (UCMJ).
The social media principles provided in this handbook are intended to outline how our core values should be demonstrated, to guide Marines through the use of social media whether personally involved or when acting on behalf of the Marine Corps.
View in its entirety (pdf) here.