If you’ve been paying any attention to Facebook for the last two weeks, you’ve probably seen dozens of friends with rainbow Facebook icons. The icons are a sweet sign of support for the recent Supreme Court case that legalized gay marriage across the nation. But we were reminded of another thing when we saw the icons–the idea of using social proof to support a cause.

What’s social proof? If you’d like the full story, you can check out the blog post we wrote on it last year. But the short version is that social proof is the idea that validation by your social network is the best way to inspire confidence and action for a brand, campaign, nonprofit, or cause. If all your friends are doing something, then you should probably do that thing too, right? (Unless, as your mother might remind you, all of your friends were jumping off a bridge.) Getting social proof grants a campaign the legitimacy of social capital–which can’t be bought.

How does this connect to the Facebook icons? The icons themselves are social proof of support of the legalization of gay marriage. When we wrote about social proof, we included a list of ten aspects of high-impact social proof. The rainbow icons comprise quite a few of those characteristics.

Which characteristics did the Facebook rainbow icons use?

• On the record: Effective social proof requires high visibility of the proof itself. Your Facebook profile picture is the visual icon of your presence on Facebook. It represents who you are–which means that changing it to indicate your support of gay marriage is a clear, public statement that follows you around every part of Facebook.

• Visual proof: A high-quality, relevant imagery boosts the impact of social proof. The rainbow profile picture itself serves as that image, and has the bonus of being personalized to the user themselves.

• Creating community: The headline on the landing page for the rainbow profile creator reads “Let’s Celebrate Pride,” which serves to include the user in the community of “us” who are celebrating Pride via Facebook.

• Using your people: The built-in social nature of Facebook means that the rainbow profile pictures are naturally exposed to the user’s friends and family. All of the pictures came tagged with a link to create your own rainbow profile picture, which creates an easy point of access for social growth.

Wondering how to instill social proof into your own campaigns? Looking at the Facebook rainbow icons–and at the 10 characteristics of social proof–would be a good place to start.