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."
Liedtke reported from San Francisco. Mae Anderson in New York contributed to this report.