By now, pretty much everyone knows about Facebook Inc.’s (NASDAQ:FB) secret mood-altering experiment. But we still don't know if the social networking giant has opened itself to a class-action lawsuit by users angry about being treated like laboratory rats.

A legal fight would be an uphill climb, but that doesn’t mean it won’t happen. “I think it would be very difficult for someone to successfully sue Facebook over this,” James Grimmelmann, a law professor at the University of Maryland, told International Business Times. “Some class-action lawyers may try in the hopes of negotiating a cash settlement, but the obstacle is the terms of service. Facebook’s terms of service say they can use data for research, and they don’t make any promises about giving you an unbiased news feed.”

Grimmelmann is one of a number of legal experts who took to the blogosphere on Monday to speculate about whether or not Facebook’s study -- which involved manipulating the newsfeeds of almost 700,000 English-speaking users for about a week in early 2012 -- violated the law. The speculation adds to the chorus of critics who already contend that the study was a violation of longstanding ethical standards in regards to experimenting on humans. At issue, Grimmelmann said, is a federal law that requires researchers receiving federal funding to obtain “informed consent” -- that is, special permission to be involved in research. Facebook users did not give informed consent, but merely agreed to a blanket clause buried in a privacy statement that few users ever read.

Facebook’s study, which was published last week in the Proceedings of the National Academy of Sciences, was authored with the help of researchers from Cornell University, which receives federal funding. But Cornell’s role in the research is somewhat murky. An initial press release suggested the research was federally funded, but that release was later amended, Grimmelmann said. In a statement on Monday, Cornell said the research was conducted independently by Facebook, and that Cornell researchers did not have access to user data or participate in the data collection.

In other words, Facebook did the dirty work, and Cornell researchers analyzed the results. As a private company, Facebook is not bound by federal law to obtain informed consent, so Grimmelmann said it likely stuck to the letter of the law -- if not the spirit of it.

Facebook has said it did not keep the information regarding which users were involved in the experiment, another aspect to the research Grimmelmann said was troubling. “We don’t know who was in the study,” he said. “We don’t know if it included people who are clinically depressed. We don’t know if it included minors. Because Facebook did it in this way, the public has no way of knowing who was affected by this and how.”

Grimmelmann isn’t the only law expert raising legal concerns. In a blog post Monday, Bradley Shear, a Maryland-based attorney who writes about social media law, wrote that Facebook’s study may have been a violation of a 2012 Federal Trade Commission order requiring it to obtain consumers’ consent before sharing their information beyond established privacy settings. Shear wrote that Facebook has “an abysmal record” when it comes to protecting its users’ privacy.

Facebook’s study, which looked at whether happy or sad posts are contagious, probably was done in part to combat the constant chatter about how seeing happy posts on Facebook is making everyone sadder. “They want the credibility of science to say Facebook is okay for you,” Grimmelmann said. “But if they want that credibility, then at the very least, they have to play by science’s rules, which involves informed consent and ethics.”

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