A UK regulator is investigating whether Facebook broke data protection laws when it conducted a psychological study on users without their consent.
The test saw Facebook "manipulate" the news feeds of nearly 700,000 users to control which emotional expressions they were exposed to.
The Information Commissioner's Office (ICO) said it planned to question Facebook over the study.
Facebook said it had taken "appropriate protections for people's information".
"We are happy to answer any questions regulators may have," Facebook's Richard Allen said in a statement.
Facebook's European headquarters are based in Dublin.
The research was conducted in collaboration with Cornell University and the University of California at San Francisco on 689,000 Facebook users over a period of one week in 2012.
According to the report on the study: "The experiment manipulated the extent to which people were exposed to emotional expressions in their News Feed".
The study found that users who had fewer negative stories in their news feed were less likely to write a negative post, and vice versa.
The research was done to gauge if "exposure to emotions led people to change their own posting behaviours".
However, the research has been criticised because Facebook users were unaware they were taking part.
Labour MP Jim Sheridan, a member of the Commons media select committee, has called for an investigation into the matter.
'Improving our processes'
For its part, Facebook has defended the study and said that there was "no unnecessary collection of people's data".
"None of the data used was associated with a specific person's Facebook account," the social networking giant said at the time.
On Tuesday, Mr Allen said: "It's clear that people were upset by this study and we take responsibility for it."
"We want to do better in the future and are improving our process based on this feedback."
Meanwhile, Adam Kramer of Facebook, who co-authored the report on the research, has admitted the firm did not "clearly state our motivations in the paper".
"I can understand why some people have concerns about it, and my co-authors and I are very sorry for the way the paper described the research and any anxiety it caused," he said earlier this week.