If you've been on Facebook in the past week, you've probably seen it: the red equals sign. Human Rights Campaign launched the movement last Monday, as the Supreme Court moved towards a ruling on Prop 8. According to Facebook, it spurred an extra 2.7 million people to change their profile pictures the following day. As political consultant Jessi Langsen of H+K Strategies told us, "It's not something we've seen before on that scale." Facebook campaigns crash and burn all the time, but this one worked, providing a popular and visible show of support at a crucial time for the issue.

Why did this campaign become a political force?

More impressive, the outcry on Facebook and elsewhere seems to have had a real impact on lawmakers. Two GOP senators have already come out in favor of gay marriage — the second just this morning. Even the Supreme Court has noticed the impact. During deliberations, Chief Justice Roberts said to a pro-marriage lawyer, "As far as I can tell, political figures are falling over themselves to endorse your side of the case."

So the question has to be asked: why? Why did this particular campaign make the leap from an online fad to a political force? What is it about gay marriage that makes it so perfect for Facebook campaigning?

"It may be social pressure, it may be mobilization, or it may be something else entirely."

The campaign's biggest asset was that it didn't directly try to change minds. The picture works as a show of support, not an entry to an argument. That plays to online activism's strengths, as most studies on the subject have shown the strongest effects coming from subtle psychological shifts. Jessica Feezell, author of a 2009 paper studying Facebook's effect on political engagement, put it this way: "Opinion change is difficult but reinforcement and mobilization are easier." The discussion may not change minds, but it can galvanize support and change the parameters of a debate.

Still, the years of studies haven't produced a clear answer on how it happens. Feezell told us the path from Like to action is "still a moving target in the research." Which is to say, we don’t really know. "It may partly be discussion, it may be a sense of affiliation, it may be social pressure, it may be mobilization, or it may be something else entirely."

The newsfeed became a forum where conservatives could test the waters

For the HRC campaign, reinforcement seems like the strongest effect. Some writers have already speculated that the campaign provided social proof, driving home how much support already existed for gay marriage. Liz Mair of consulting firm Mair Strategies noticed an especially powerful effect in conservative circles. "I know several conservatives who silently supported same-sex marriage," Mair told The Verge, "but until they saw other right-of-center friends changing their profile pictures, they didn't know that support was so widespread." Once they'd seen support expressed on Facebook, Mair suspects, they were more willing to express support in a fully public forum. "It encouraged them to come out, so to speak, as same-sex marriage supporters."

The limited scope of Facebook, speaking only to a network of friends, provided a way of going public without going public. The newsfeed became a semi-private forum where conservatives could test the waters without risking a truly public backlash. A Facebook picture isn't a broadcast, like a tweet or Tumblr post would be, so a changed profile pic was more akin to voicing support at a small dinner party. It's easy to comment and Like a changed profile picture — giving it the viral spread that attracts marketers and activists alike — but arguing with a changed profile picture feels more aggressive than responding to a text post, so HRC ended up with the best of both worlds. And because there’s no "dislike" button, Edgerank ends up surfacing more support than criticism.

A Facebook campaign also reaches across demographic lines in a unique way. As Langsen points out, "Facebook is a platform where several generations are now very active at once." In an era when most political discussion takes place in small groups of like-minded individuals, Facebook exposes older generations to the views of younger relatives they might never have otherwise encountered.

Facebook made a difference by doing what Facebook does best: connecting people

With a successful enough campaign, the newsfeed becomes a machine for Rob Portman moments, exposing skeptics to distant relatives and old friends they may have forgotten. Out of the average user's 190 friends, one of them is bound to have a personal stake in the issue — and as an oft-cited Gallup poll points out, simply knowing a gay person can be the most important factor in building support for gay marriage.

In other words, Facebook made a difference by doing what Facebook does best: connecting people. And gay marriage is the rare issue where simply connecting people can make a huge difference in public opinion. Experienced analysts will tell you that the personal is always political, but HRC's campaign managed to make the connection vitally clear. When Chief Justice Roberts talks about the shocking "political force and effectiveness" of the gay marriage movement, he means the red equals sign. It's just people, saying other people are okay. In the age of social media, that’s a powerful thing.