According to BBC News Technology, "Facebook is facing criticism after it emerged it had conducted a psychology experiment on nearly 700,000 users without their knowledge."

The test saw Facebook "manipulate" news feeds to control which emotional expressions the users were exposed to. The research was done in collaboration with two US universities to gauge if "exposure to emotions led people to change their own posting behaviours".

Facebook said 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 added.

Cornell University and the University of California at San Francisco were involved in the study.

The research was conducted 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.

Adam Kramer of Facebook, who co-authored the report on the research, said: "We felt that it was important to investigate the common worry that seeing friends post positive content leads to people feeling negative or left out".

"At the same time, we were concerned that exposure to friends' negativity might lead people to avoid visiting Facebook." However, he admitted that 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."