It's emerged that Facebook "manipulated" the amount of positive and negative news in the feeds of almost 700,000 people as an experiment. Here's a taster of the reaction on social media.
"I am NOT a lab rat!"
"Creepy and disturbing"
"This is bad, even for Facebook."
As the tweets above show, many are clearly outraged on hearing news that Facebook - together with Cornell University and the University of California - carried out a study to see if people's moods were affected by reading a positive, or negative newsfeed. The answer appears to be "yes", although the effect isn't huge.
According to the study, which was published earlier this month, people reading more positive newsfeeds used very slightly more positive words themselves on Facebook, and vice-versa.
What has angered many Facebook users is that none of those taking part in the test were told they were being experimented on.
Facebook says this is perfectly legal under their terms of service. But, following the furore, Adam Kramer, one of the Facebook scientists involved, wrote in a Facebook post that he was "sorry for the way the paper described the research and any anxiety it caused".
Some commenting on the BBC World Facebook page didn't see what the fuss was about. "I think Facebook would be stupid not to do it," wrote Andrew Farley. "What an opportunity to learn about humanity."
But a few threatened to leave Facebook. "We should conduct a mood experiment on Facebook. 1. We all leave. 2. Someone asks them how they feel about it," was one tweet for example. Whether many will follow through on that threat is unclear. Back in 2010, a "Quit Facebook Day" was organised in protest at the company's privacy policies, but was widely regarded as a flop.
"We are reliant upon these technology platforms, and we cannot easily give them up," says Professor Ralph Schroeder at the Oxford Internet Institute. Schroeder calls the Facebook test "very troubling". "If this had been a study conducted within academia, I doubt very much it would have got ethical approval," he told BBC Trending.
Most importantly, he says, it shows how powerful "big data" is. It's not too much of a leap, he says, to imagine a "brave new world" where social platforms, governments or others, might try to condition our feelings and emotions - without us even knowing. Academics and regulatory bodies need to monitor this closely, he says.
Reporting by Cordelia Hebblethwaite
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