By Monday, hundreds of thousands of people had checked in at Standing Rock Indian Reservation on the social networking site.
But many of them weren't anywhere near the location where demonstrators have been picketing the controversial $3.7 billion pipeline.
A post circulating on Facebook gave one possible explanation for the surge in activity, claiming that the mass check-ins were organized to prevent local law enforcement from tracking protesters on social media.
The sheriff's department denied that accusation on Monday, calling it "absolutely false."
