In the summer of 2015, as Memphis exploded with protests over the police killing of a 19-year-old man, activists began hearing on Facebook from someone called Bob Smith. His profile picture [was] a Guy Fawkes mask, the symbol of anti-government dissent. Smith acted as if he supported the protesters. Over the next three years, dozens of them accepted his friend requests, allowing him to observe private discussions. He described himself as a far-left Democrat, a fellow protester" and a "man of color." But Smith was not real. He was the creation of a white detective in the Memphis Police Department's Office of Homeland Security whose job was to keep tabs on local activists. The detective, Tim Reynolds, outed himself in August under questioning by the American Civil Liberties Union of Tennessee, which sued the police department for allegedly violating a 1978 agreement that prohibited police from conducting surveillance of lawful protests. The revelation validated many activists' distrust of local authorities. It also provided a rare look into the ways American law enforcement operates online. Social media monitoring - including the use of software to crunch data about people's online activity - illustrates a policing "revolution" that has allowed authorities to not only track people but also map out their networks, said Rachel Levinson-Waldman, senior counsel at [the] Brennan Center for Justice. But there are few laws governing this kind of monitoring.