“It's not an experiment or test,” Nevius told me of the rainbow feature. “Everyone sees the same thing.”

But all this raises a serious question: Is Facebook doing research with its “Celebrate Pride” feature? Facebook's data scientists have attracted public scrutiny for conducting experiments on its users: tracking their moods and voting behavior. Much less attention has been given to their ongoing work to better understand collective action and social change online.

In March, the company published a paper that got little outside attention at the time, research that reveals some of the questions Facebook might be asking now. In “The Diffusion of Support in an Online Social Movement,” Bogdan State, a Stanford Ph.D. candidate, and Lada Adamic, a data scientist at Facebook, analyzed the factors that predicted support for marriage equality on Facebook back in March 2013. They looked at what factors contributed to a person changing their profile photo to the red equals sign, but the implication of their research is much larger: At stake is our understanding of whether groups of citizens can organize online—and how that collective activity affects larger social movements.

Scholars and activists have debated the effectiveness of profile-image campaigns since at least 2009, when Twitter users turned their profiles green, joined Facebook groups, and changed their location setting to Tehran in support of Iranian protesters. Experts downplayed the importance of such actions; Global Voices Iran editor Fred Petrossian argued that talk of a Twitter revolution "reveals more about Western fantasies for new media than the reality in Iran." Evgeny Morozov, who was a Yahoo fellow at the time, called it “slacktivism,” a “harmless activism” that “wasn't very productive.”

Among other critiques, Morozov voiced two important questions in a larger debate over the value of collective action online. First, he argued that social-media solidarity has an unknown effect toward political change, perhaps even siphoning energy away from more effective action. Secondly, Morozov downplayed the cost and risk of that participation. But unlike Westerners showing solidarity for Iranians on Twitter, gender equality in the U.S. involves changes in social relations alongside political changes. Changing one’s profile image in support of marriage equality in America carries immediate risks and costs, from “a quarrel with one’s otherwise-thinking friends—to the life-threatening,” as State and Adamic explain in their research.

Indeed, it's hard to look at the compilation of coming-out videos posted by YouTube on Friday and dismiss online activity as inconsequential. The “slacktivism” of changing a profile image matters in part because of the personal risks it may entail, and in part because it may contribute to changes in the social acceptance of LGBTQ people. While some might argue a rainbow-colored profile is a lazy way of showing support, it could be an act of great courage for the person changing their profile.

What leads people to participate in costly, risky social change, anyway? And might people be more likely to get involved if their friends also participate? That's the question asked by Stanford sociologist Doug McAdam in his research on Freedom Summer, a 1964 civil-rights action that placed over 700 college students in black families’ homes in Mississippi to register black voters. Over 10 weeks, there were three murders, “52 serious beatings, 250 arrests, and 13 black churches burned to the ground.” When McAdam found an archive of 1,086 Freedom Summer volunteer application forms, including lists of volunteers' most trusted friends, he had a unique opportunity to test the factors associated with costly, risky social change.

In a statistical model, McAdam found that the greatest predictors of involvement in Freedom Summer were “(a) greater number of organizational affiliations, (b) higher levels of prior civil rights activity, and (c) stronger and more extensive ties to other participants.” In other words, among those who applied and were accepted, people with more participating friends were more likely to go through with their activism, when controlling for other factors. And when Facebook’s researchers studied how support for marriage equality spread on their social network, they cited McAdam’s research and Freedom Summer as an important inspiration.

Although McAdam later studied network structure, The Freedom Summer data couldn’t answer the question of whether seeing others take action prompts a person to get involved. But in March 2013, when millions of Facebook users changed their profile, Facebook’s researchers saw it as a chance to evaluate how participation spreads.

In their study, State and Adamic asked the question: how many times do you need to see a friend change their profile picture before deciding to change your own? They set up two competing hypotheses. The first possibility was that profile changes spread like funny pictures and other online memes, falling off in influence as more people share them. The second possibility they considered was that people need to see others make the change before they follow suit, that “multiple exposures are most effective in determining the adoption of... [costly] behaviors.”

To test these competing hypotheses and develop a new model for how solidarity spreads from person to person, Facebook’s researchers classified profile images from over 3 million users in March 2013, along with 106 million users who were exposed to those changed profiles. Next, they predicted the likelihood of someone changing their profile to an equality image, depending on how many friends they had seen make the change. State and Adamic found that while someone’s likelihood to participate varied based on several factors—a person’s political affiliations, religion, and age, for example—the likelihood to change one’s profile image was greater with more exposures to changes by friends. According to State and Adamic, this likelihood increased “only for the first six exposures.” After the sixth exposure, the relationship “becomes virtually flat.”