The US government might now weigh in on Facebook's secret 2012 study on the moods of nearly 700,000 users.

Sen. Mark R. Warner (D-Va.) has penned a letter (PDF) to the Federal Trade Commission asking the regulatory agency to investigate the issue. He requested that the FTC look into the ramifications of the experiment and to consider whether rules should be put in place for such types of experiments on social networks.

"I understand that social-media companies are looking for ways to extract value from the information willingly provided by their huge customer base," Warner said in a statement. "But I think many consumers were surprised to learn they had given permission by agreeing to Facebook's terms of service. And I think the industry could benefit from a conversation about what are the appropriate rules of the road going forward."

Facebook has faced a steady stream of backlash since word spread a couple of weeks ago that it conducted the emotion manipulation study -- unbeknownst to users -- in January 2012.

The study, called "Experimental Evidence of Massive-Scale Emotional Contagion Through Social Networks," was carried out by researchers from Facebook, Cornell University, and the University of California at San Francisco. During the course of the study, Facebook altered the News Feeds of 689,003 random users to show either more positive or more negative posts -- and thus possibly altered their emotional states.

The goal of the study was to see whether happiness or negativity could be contagious on the social network, and the researchers concluded that emotions were indeed transmittable.

Critics say the social network lacks transparency and that it was unethical to fiddle with user data without alerting people first. People less worried about the issue say Facebook meddles with user data all the time with its advertising and marketing testing.

In his letter to the FTC, Warner didn't necessarily criticize Facebook for conducting the study -- he just said he wants to know more about consumers' right to privacy on social networks. He also asked the FTC whether it believes federal authorities or the tech industry itself should develop possible oversight practices on these types of experiments.

"Future studies like this, without proper oversight or appropriate review, could have a significant impact upon a large number of consumers," Warner wrote in his letter. "The very fact that important questions remain unanswered highlights the lack of transparency around these business practices. For example, while Facebook may not have been legally required to conduct an independent ethical review of this behavioral research, the experiment invites question about whether procedures should be in place to govern this type of research."

If the FTC decides to get involved, this wouldn't be the first time it's gone head-to-head with Facebook over privacy issues. In 2011, the social network settled a complaint with the FTC that claimed the company "deceived" users by altering its privacy practices without warning. Under the terms of the settlement, Facebook agreed to let users "opt in" to changes that alter how personal information is shared with advertisers and other users.

When contacted by CNET, a Facebook spokesperson said the social network will answer any questions from the FTC.

"It's clear that people were upset by this study and we take responsibility for it," the spokesperson said. "We want to do better in the future and are improving our process based on this feedback. The study was done with appropriate protections for people's information and we are happy to answer any questions regulators may have."