STUDIO CITY (CBSLA.com) — Facebook is coming clean about a social experiment it conducted on nearly 700,000 of its users two years ago.

The popular social media platform manipulated people’s Facebook news feeds for one week in 2012 to find out whether increasing the amount of positive or negative messages would have an effect on the user.

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Researchers found that when people saw fewer positive posts, they wrote fewer positive comments of their own.

Paul Royer, a clinical social worker, called the test “manipulative.”

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“There’s this constant flow of information, so when that information can be manipulated, it just tells us how vulnerable people really are,” Royer said.

The study has revived privacy concerns on Facebook.

Deep in Facebook’s terms, however, the network says it can utilize users’ information for research.

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In a statement, Facebook said: “We do research to improve our services and to make the content people see on Facebook as relevant and engaging as possible. A big part of this is understanding how people respond to different types of content, whether it’s positive or negative in tone.”