Facebook "communicated really badly" about a controversial study in which it secretly manipulated users' feelings, the social network's chief operating officer Sheryl Sandberg admitted Wednesday.
The company clandestinely altered the emotional content of feeds of nearly 700,000 users for a week in 2012, giving some sadder news and others happier news in a study aimed at better understanding "emotional contagion".
The research, published last month, has prompted online anger and questions about the ethics of the research and forced Facebook on the defensive.
It was an experiment as part of product testing, 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."
Sandberg, who was in India to promote her gender-equality book "Lean In" and meet leaders of Indian companies and senior politicians, declined to speak to reporters asking further questions.
The comments came as several European data protection regulators began looking into whether Facebook broke privacy laws when it carried out the study.
British authorities will question Facebook over the experiment, officials said Wednesday.
The Information Commissioner's Office, Britain's independent data watchdog, is liaising with the Irish data protection authority and seeking "to learn more about the circumstances", a spokesman said.
The study by researchers affiliated with Facebook, Cornell University and the University of California at San Francisco, appeared in the June 17 edition of the Proceedings of the National Academy of Sciences.
In the study, Facebook placed positive or negative posts in users' feeds to see how this affected their mood -- all without their explicit consent or knowledge.
The results indicate "emotions expressed by others on Facebook influence our own emotions, constituting experimental evidence for massive-scale contagion via social networks", the researchers concluded, and noted emotion was relevant to human health.
Given Facebook's billion users and widespread influence, the experiment has raised worries over its ability to influence users' moods and thinking.
Critics say research on people is normally governed by strict ethical regulations.
In a statement earlier in the week, Facebook said none of the data used in the study was associated with a specific person's account.
It said it did research to make its content "as relevant and engaging as possible", which meant understanding how people respond to positive or negative information.
The researchers said the study was consistent with Facebook's Data Use Policy, to which all users agree before creating a Facebook account.
But a number of users criticised the psychological experiment, posting words like "super disturbing," "creepy" and "evil" as well as angry expletives.
The study can be found at: www.pnas.org/content/111/24/8788.full.pdf