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

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 U.S. universities to gauge if "exposure to emotions led people to change their own posting behaviors".

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, according to the BBC.

Cornell University and the University of California at San Francisco were involved in the study. But some have criticized the way the research was conducted and raised concerns over the impact such studies could have.

However, Katherine Sledge Moore, a psychology professor at Elmhurst College, Illinois, said: "Based on what Facebook does with their newsfeed all of the time and based on what we've agreed to by joining Facebook, this study really isn't that out of the ordinary. The results are not even that alarming or exciting."

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."