Facebook barred software developers on Monday from using its massive data pool to create surveillance tools, closing off a process that had been exploited by U.S. police departments to track protesters.

Facebook, Instagram, and Twitter were accused last year by privacy advocates after the American Civil Liberties Union (ACLU) said in a report that police were using location data and other user information to spy on protesters in places such as Ferguson, Missouri.

In response to the ACLU report, the companies shut off the data access of Geofeedia, a Chicago-based data vendor that said it works with organizations to “leverage social media,” but the Facebook policy had not explicitly barred such use of data in the future.

“Our goal is to make our policy explicit,” Rob Sherman, Facebook’s deputy chief privacy officer, said in a post on the social network on Monday. He was not immediately available for an interview. The change would help build “a community where people can feel safe making their voices heard,” Sherman said.

Monitoring of social media by law enforcement is a growing widespread practice and given its low cost compared to other forms of surveillance, the adoption of these tools may not come as a surprise. Social media data has helped law enforcement agents in solving murder cases, detect human trafficking activities and more. But there is a growing concern over how social media affects First Amendment rights which protect free speech and Fourth Amendment rights which protect against unreasonable search and seizure.

Racially charged protests broke out in the St. Louis suburb of Ferguson in the aftermath of the August 2014 shooting of Black teenager Michael Brown by a white police officer. The groups argue that social surveillance tools have a negative effect on the freedom of speech. People are less likely to speak out, if aware that the police will start watching every move made in response. There’s a concern that you may be falsely implicated in criminal behaviour simply because you posted in the same general location as a handful of protesters that broke the law.

Major social media platforms including Twitter and YouTube have taken action or implemented policies similar to Facebook’s, said Nicole Ozer, technology, and civil liberties policy director at the ACLU of Northern California. Ozer praised the companies’ action but said they should have stopped such use of data earlier. “It shouldn’t take a public records request from the ACLU for these companies to know what their developers are doing,” she said.

It was also unclear how the companies would enforce their policies, said Malkia Cyril, executive director of the Center for Media Justice, a non-profit that opposes government use of social media for surveillance. Inside corporations, “Is the will there, without constant activist pressure, to enforce these rules?” Cyril said.

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