Protesters at the Standing Rock Sioux Reservation in Cannon Ball, N.D., rallying against the Dakota Access oil pipeline have been calling for reinforcements since summertime. As of this week, about 800 people had come. But in the course of just a few days, more than 1.5 million people marked themselves present at the pipeline protest using Facebook - even though they weren't actually there. A recent report by the American Civil Liberties Union revealed that law enforcement agencies across the country use location tracking and social media data to identify activists. Protesters have reported phones turning on on their own, phone calls cutting out and live video streams being interrupted as evidence that they're being spied on, said Jennifer Cook, the policy director for the ACLU of North Dakota. Law enforcement agencies ... said they are not relying on Facebook's check-in system to track protesters. Last week, video of violent clashes between lines of police and protesters circulated online, showing demonstrators running from officers as 142 people were arrested. Most of them were charged with rioting and criminal trespassing. About 300 people have been arrested since the protests began over the summer. Tensions have intensified in recent weeks as the pipeline's construction moves closer to a river crossing that activists view as a critical water source that they fear will be compromised by the oil main, which many Native American tribes have said treads on sacred land.