NEW YORK: The revelation that Facebook, the social networking giant, manipulated the news feeds of more than half a million users in order to examine how emotions can be spread on social media has sparked widespread anger.

Two academics, from the University of California and Cornell University, teamed up with a Facebook researcher to filter the news feeds of a random selection of users. Some were given reduced exposure to their friends' "positive emotional content", others reduced exposure to friends' "negative emotional content". In the first case, test users made fewer positive posts of their own, in the second they made fewer negative posts.

"These results indicate that emotions expressed by others on Facebook influence our own emotions, constituting experimental evidence for massive-scale contagion via social networks," the study said.

But the fact that Facebook users were not informed of the 2012 experiment means that ethical and legal guidelines may have been breached according to some observers. Facebook itself claims that users consent to such research when they agree to its terms of service.

"People are supposed to be told they are going to be participants in research and then agree to it and have the option not to agree to it without penalty," Susan Fiske, professor of psychology and public affairs at Princeton University, told the Guardian. And James Grimmelmann, professor of law at Maryland University, described the study as "a scandal" that brought Facebook's "troubling practices" into the academics realm.

Even if Facebook hadn't done anything actually illegal "they didn't do right by their customers," Brian Blau, a technology analyst with research firm Gartner, told the New York Times. "Doing psychological testing on people crosses the line," he added.

Others were concerned at the wider implications of the research findings. "Facebook now knows it should subject you to emotional steroids to keep you coming back," said Forbes.

That might be an overstatement as the Facebook researcher involved noted, in an apology post, that "the actual impact on people in the experiment was the minimal amount to statistically detect it — the result was that people produced an average of one fewer emotional word, per thousand words, over the following week".

Whatever the rights and wrongs of this particular case, it adds to a growing sense of concern in some quarters about how internet companies treat their users. "If they don't get the value exchange right then people will be reluctant to use their services, which is potentially a big business problem," observed Robert Blackie, director of digital at Ogilvy One marketing agency.

Data sourced from New York Time, Forbes, The Guardian; additional content by Warc staff