Facebook's newsfeed study isn't just controversial among Internet users and academics, it turns out. Now, even the journal that published Facebook's research says it has reservations about having done so.

The Proceedings of the National Academy of Sciences, one of the top scientific journals in the country, said Thursday it was publishing an editorial expression of concern regarding Facebook's study.

Although Facebook didn't technically break any rules on human-subject research, the journal said, Facebook's research "may have involved practices that were not fully consistent with the principles of obtaining informed consent and allowing participants to opt out."

Expressions of concern are a way for scholarly journals to notify readers of potential problems in published research, though PNAS said it did not believe Facebook needed to comply with the paper's human-subject research policies.

PNAS added in a statement to the Washington Post that the announcement was aimed at acknowledging concerns about the research and that it does not intend to investigate the study further.

Facebook has come under sustained criticism in recent days for the study. In the experiment, researchers tweaked the newsfeed for a random group of nearly 700,000 users in an attempt to determine whether different emotional tones in the newsfeed would drive users away from the service.

"When someone signs up for Facebook, we've always asked permission to use their information to provide and enhance the services we offer," said Facebook spokeswoman Jodi Seth. "To suggest we conducted any corporate research without permission is complete fiction ... We are taking a very hard look at our review process to make improvements."

Facebook chief operating officer Sheryl Sandberg has apologized for having "poorly communicated" the study to Facebook users, but did not apologize for having conducted the study itself. Sandberg added that the kind of product testing involved in the study happens all the time. Others (including me) have made much the same point.

The controversy has already led consumer groups to take legal action, with the Electronic Privacy Information Center filing a complaint with the Federal Trade Commission Thursday.

"The company purposefully messed with people’s minds," the complaint reads. "At the time of the experiment, Facebook did not state in the Data Use Policy that user data would be used for research purposes. Facebook also failed to inform users that their personal information would be shared with researchers.”

Read PNAS' full expression of concern below.