For the last three years, U.S. Central Command (which controls American forces in the Middle East and Afghanistan) has used a Digital Engagement Team (DET) to monitor the Internet for misinformation about American military operations, or outright lies and slander. The DET personnel then post an official rebuttal. DET contains staff fluent in Arabic, Dari, Persian, Pashto, Urdu and Russian (still used by older folks who came of age when Russian foreign aid was abundant in the region).
Other branches of the U.S. government engage less openly, and use hackers to get into hostile sites, to find out who is running them, or to insert disruptive messages. Intelligence agencies often do this, and also use native speakers of Middle Eastern languages to covertly engage anti-American talk on message boards and chat rooms. But most of those doing this sort of thing are not U.S. government employees, but migrants from the Middle East, living in the United States and hostile to the lies being posted about their new home, and Moslems who have moved there.