The Information Commissioner's Office said Wednesday that it wants to learn more about the circumstances underlying a 2-year-old study carried out by two U.S. universities and the world's largest social network.
The inquiry is being coordinated with authorities in Ireland, where Facebook has headquarters for its European operations, as well as with French regulators.
This is just the latest in a string of incidents that have raised questions about whether the privacy rights of Facebook's nearly 1.3 billion users are being trampled by the company's drive to dissect data and promote behavior that could help sell more online advertising.
In this case, Facebook allowed researchers to manipulate the content that appeared in the main section, or "news feed," of about 700,000 randomly selected users during a single week in January 2012. The data-scientists were trying to collect evidence to prove their thesis that people's moods could spread like an "emotional contagion" depending on the tenor of the content that they were reading.
The study concluded that people were more likely to post negative updates about their lives after the volume of positive information appearing in their Facebook feeds had been purposefully reduced by the researchers. The opposite reaction occurred when the number of negative posts appeared in people's news feeds.
Facebook's data-use policy says the Menlo Park, California, company can deploy user information for "internal operations, including troubleshooting, data analysis, testing, research and service improvement."
The reaction to the study itself provided evidence of how quickly an emotional contagion can spread online. The research was released a month ago, but it didn't provoke a backlash until the past few days after other social media sites and essays in The New York Times and The Atlantic raised red flags about the ethics of Facebook's experiment.
As it has done in several past breaches of privacy etiquette, Facebook is now trying to make amends.
Sheryl Sandberg, Facebook's chief operating officer, told television network NDTV in India that "we clearly communicated really badly about this and that we really regret." Later she added: "Facebook has apologized and certainly we never want to do anything that upsets users."
The words of contrition sounded hollow to Jeff Chester, executive director of the Center for Digital Democracy, a privacy-rights group. He points to Facebook job openings looking for researchers specializing in data mining and analysis as evidence that the company still has every intention of digging deeper into its users' psyches and preferences.
"They are engaged in secret surveillance of its users to figure out how to make more money for their advertisers," Chester said.
Whatever Facebook has been doing has been paying off for the company and its shareholders. Facebook's revenue last year rose 55 percent to $7.9 billion and its stock has nearly tripled in value during the past year.
The concern over Facebook's experiment comes amid interest in Europe about beefing up data-protection rules. The European Court of Justice last month ruled that Google must respond to users' requests seeking to remove links to personal information.
Suzy Moat, a Warwick Business School assistant professor of behavioral science, said businesses regularly do studies on how to influence behavior. She cited the example of Facebook and Amazon experimenting with showing different groups of people slightly different versions of their websites to see if one is better than another at getting customers to buy products.
"On the other hand, it's extremely understandable that many people are upset that their behavior may have been manipulated for purely scientific purposes without their consent," Moat said. "In particular, Facebook's user base is so wide that everyone wonders if they were in the experiment."