To Facebook, we are all lab rats.

Facebook routinely adjusts its users’ news feeds — testing out the number of ads they see or the size of photos that appear — often without their knowledge. It is all for the purpose, the company says, of creating a more alluring and useful product.

But last week, Facebook revealed that it had manipulated the news feeds of over half a million randomly selected users to change the number of positive and negative posts they saw. It was part of a psychological study to examine how emotions can be spread on social media.

The company says users consent to this kind of manipulation when they agree to its terms of service. But in the quick judgment of the Internet, that argument was not universally accepted.

“I wonder if Facebook KILLED anyone with their emotion manipulation stunt. At their scale and with depressed people out there, it’s possible,” the privacy activist Lauren Weinstein wrote in a Twitter post.