Have you read Facebook’s Terms of Service? If not, read on

UPDATED on July 1 at 2:05 p.m. EDT: Facebook users discovered this weekend that their News Feeds may have been altered for a psychological experiment.

According to a June 17 paper in the Proceedings of the National Academy of Science, 689,000 English-language Facebook users were included in an experiment meant to test whether “in-person interaction and nonverbal cues are not strictly necessary for emotional contagion.”

To test the theory, Facebook removed negative posts from one group of users’ News Feeds, and positive posts from another.

On Sunday, Facebook’s data scientist behind the study, Adam Kramer, posted an explanation on his Facebook page. Kramer said that posts weren’t “hidden” from News Feeds, they just weren’t showing up. Meaning? You could still see a friend’s post if you went directly to their page. But if you relied on your News Feed for their latest life update, you might be missing something.

“And at the end of the day, the actual impact on people in the experiment was the minimal amount to statistically detect it — the result was that people produced an average of one fewer emotional word, per thousand words, over the following week.”

The Terms of Service you agree to when you sign up for Facebook currently includes a section entitled “Information we receive and how we use it.” That grants Facebook permission to use your data for the likes of “data analysis, testing, research and service improvement.”

But that policy was altered four months after Facebook’s study took place in May 2012 to include that line about how your information might be used. According to Forbes, the prior Terms of Service version from September 2011 “did not mention anything about user information being used in ‘research.'”

We’re asking: Did you read Facebook’s Terms of Service? Do you know what it is you’re agreeing to when you sign up for a social media network? Tell us in the comments below.