It's recently been revealed that the U.S. government contracted HBGary Federal for the development of software which could create multiple fake social media profiles to manipulate and sway public opinion on controversial issues by promoting propaganda. It could also be used as surveillance to find public opinions with points of view the powers-that-be didn't like. It could then potentially have their "fake" people run smear campaigns against those "real" people. As disturbing as this is, it's not really new for U.S. intelligence or private intelligence firms to do the dirty work behind closed doors.
EFF previously warned that Big Brother wants to be your friend for social media surveillance. While the FBI Intelligence Information Report Handbook (PDF) mentioned using "covert accounts" to access protected information, other government agencies endorsed using security exploits to access protected information.