Facebook communicated "terribly" about a controversial study in which it secretly manipulated users' feelings, the social network's chief operating officer Sheryl Sandberg admitted Wednesday.
Sandberg's comments came as British authorities said they would question Facebook over the experiment to see whether it broke privacy laws.
The research 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.
"This was communicated terribly and for that communication we have apologised," she said.
"We communicated really badly on this subject," she said, before adding: "We take privacy at Facebook really seriously."
Facebook clandestinely altered the emotional content of feeds of nearly 700,000 users, giving some sadder news and others happier news in the 2012 study aimed at better understanding "emotional contagion".
The research, published last month, has prompted online anger and questions about the ethics of the week-long study, and put the world's most popular networking site on the defensive.
'Endless opportunity' in India
Sandberg is in India to promote her gender equality book "Lean In", meet business people and political leaders and scout for new business opportunities.
The Facebook executive declined to speak to reporters asking further questions about the study.
Britain's independent data watchdog, the Information Commissioner's Office, is liaising with the Irish data protection authority and seeking "to learn more about the circumstances" of the study, a spokesman said.
Facebook's European headquarters are based in the Irish capital Dublin, meaning EU laws, not US ones, apply to its operations there.
"We work very closely with the regulators all over the world ... we are fully compliant (with regulations)," Sandberg said.
In the study, Facebook placed positive or negative posts in users' feeds to gauge how this affected their mood - 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", researchers concluded, and noted emotion was relevant to human health.
The psychological experiment has stoked worries over the mood-altering capacities of the site, which has 1.2 billion users. 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".
But some users have criticised the experiment, describing it as "creepy", "evil", and "super disturbing".
Sandberg said that India - which has over 100 million Facebook users, the most after the US - is poised to become the site's largest market.
Local media reported she would meet Thursday with Prime Minister Narendra Modi, who used social media extensively in his recent election campaign and has 18 million fans on Facebook.
"There are a billion people in India who haven't connected," said Sandberg, 44, whose Facebook stake helped make her one of the world's youngest billionaires.
Sandberg, who worked in India in the early 1990s on a World Bank leprosy project, called the country an "endless opportunity".
The executive, whose profile has been raised by best-seller "Lean In", brushed aside questions about whether she might run for political office.
"I'm doing all the 'leaning in' I can," she said.
But she added with just 18 women heading countries, it is vital for more women to succeed in politics.
"We are actually cheating our economic potential, as we are not using the full power of the population," she said.