An estimated three million Facebook users changed their profile pictures to a red equals sign or rainbow - the logo used by the Human Rights Campaign-in celebration of the controversial decision by the U.S. Supreme Court to allow gay marriages across the country on Friday.

What they do not know was the rainbow filter was actually a Facebook psychological experiment to determine how to influence their behavior, claimed MIT network scientist Cesar Hidalgo.

On his Facebook page, Hidalgo wrote, "This is probably a Facebook experiment! "The question is, how long will it take for people to change their profile pictures back to normal."

The Conservative Post said this was not the first time Facebook conducted this experiment that provided them an unprecedented insight on how to influence their users. In 2013, Facebook published a result of a study they conducted to determine what will influence users to change their profile picture to support gay marriage.

The report also quoted Suzy Moat, a Warwick Business School assistant professor of behavioral science, who said that businesses like Facebook normally conducts research studies on how to influence market behavior. But she said such studies could backfire as some users are expected to detest such experiments.

She said, "On the other hand, it's extremely understandable that many people are upset that their behavior may have been manipulated for purely scientific purposes without their consent. In particular, Facebook's user base is so wide that everyone wonders if they were in the experiment."

Facebook has denied the rainbow filter was part of a behavioral experiment. A spokesman for the social media site told the Daily Mail, "This was not an experiment or test, but rather something that enables people to show their support of the LGBTQ community on Facebook. The point of this tool is not to get information about people."

In another report, the Atlantic claimed Facebook has long been undertaking research to have a better profile of how information spreads in a social media. In March 2013, Facebook conducted a research with the aim of developing a new model for how solidarity spreads from person to person.

Meanwhile, some conservative Americans have launched a campaign asking users to change their profile pictures with an American flag or any other photo to combat the three million people who are now using rainbow filters on their Facebook profile photos in support of gay marriage, reported Gizmodo.

Conservative and devout Christians who are against the Supreme Court ruling are posting the stars and stripe across their avatars saying that the American flag, now more than ever, stans for equal rights.