British authorities said Wednesday they will investigate Facebook over an experiment which manipulated the feelings of users, as the social network apologised for its poor handling of the row.
Facebook clandestinely altered the emotional content of news feeds of nearly 700,000 users for one week in 2012 without their knowledge, in order to test whether it altered their moods.
News of the "creepy" experiment has caused outrage among users, and on Wednesday Britain's independent data watchdog, the Information Commissioner's Office, said it was now looking into the case.
"We're aware of this issue and will be speaking to Facebook, as well as liaising with the Irish data protection authority, to learn more about the circumstances," a spokesman told AFP.
Facebook, the world's most popular social networking site with 1.2 billion users, has its European headquarters in Dublin.
As the row grew, Facebook's chief operating officer Sheryl Sandberg admitted during a visit to India on Wednesday that the company had communicated badly on the experiment.
"This was communicated terribly and for that communication we have apologised," Sandberg told a women's business seminar in New Delhi when asked whether the study was ethical.
"We communicated really badly on this subject," she said, before adding: "We take privacy at Facebook really seriously."
The research, published last month, involved Facebook giving some users sadder news and others happier news in order to better understand "emotional contagion".
Researchers wanted to see if the number of positive or negative words in messages the users read determined whether they then posted positive or negative content in their status updates.
It did not seek explicit consent beforehand, but claims its terms of service contract with users permits blanket "research".
Users, however, questioned the ethics of the study with some calling it "super disturbing", "evil" and "creepy".
Facebook said the company was "happy" to answer the British regulator's questions.
"It's clear that people were upset by this study and we take responsibility for it," a Facebook spokesman told AFP by email.
"We want to do better in the future and are improving our process based on this feedback.
"The study was done with appropriate protections for people's information and we are happy to answer any questions regulators may have."
Sandberg, who was is in India to promote her gender equality book "Lean In", gave a more general assurance about Facebook's cooperation.
"We work very closely with the regulators all over the world... we are fully compliant (with regulations)," Sandberg said.
She declined to speak to reporters asking further questions about the study.
The experiment was conducted by researchers affiliated with Facebook and Cornell University and the University of California at San Francisco in the United States.
The results indicate "emotions expressed by others on Facebook influence our own emotions, constituting experimental evidence for massive-scale contagion via social networks", researchers concluded, and noted emotion was relevant to human health.
The psychological experiment has stoked worries over the mood-altering capacities of the site. Critics say research on people is normally governed by strict ethical regulations.
In a statement earlier in the week, Facebook said the study was consistent with its blanket data use policy, to which all users agree. It said it does research to make its content "as relevant and engaging as possible".