The Facebook check-in tool is being used en-masse to protest against the construction of the controversial Dakota Access Pipeline through land deemed sacred by Native American Indians.
More than a million people have checked-in at Standing Rock Indian Reservation, which is the site of ongoing protests against the construction of a $3.7 billion gas pipeline through the reserve considered sacred by Native American Indians.
The sudden check-in influx came following the circulation of a post which called on people to use the function at 'Standing Rock' as a way of countering what it called police "attempts to disrupt the prayer camps" by "targeting" protesters.
The Morton County Sheriff's Department responded by calling the accusations "absolutely false" in a Facebook post.
Protests, which date back to 2014, relate to the pipeline's location under the Missouri River, the primary drinking water source for the Standing Rock Sioux, a Native American tribe of around 10,000 people.
The pipeline is a project of the Texas-based Energy Transfer Partners project, and if completed, would cross four US states.
People continue to check in from across the US and worldwide.