So we've learned something about what matters to the user from the people (or lack thereof) in their profile picture. But a profile picture can also be used as a tool to send a message. 

12 people uploaded profile pictures with the rainbow stripe to celebrate the Supreme Court decision in favor of same-sex marriage. By altering their profile picture, these users publicly show their support for gay pride.

15 of the profile pictures I examined were advertisements of some kind. Many were sorority or fraternity paraphernalia, announcements to publicize upcoming events, or notices about joining various committees or clubs. These users know that their profile pictures are prominent on social media sites and will be seen by many of their peers, so they take advantage of the change to promote something they are a part of.

And while these profile pictures send a message - supporting a cause or publicizing an event or group - they still, at the core, are used to reveal something about what matters to the user. We learn what causes they are passionate about, what groups they are involved with, or what events they plan to attend. And a profile picture is not used to show just any old thing they care about. Rather, it's something so prominent and so important in their life that they chose that single thing to represent their entire profile - and thus their entire online identity - on Facebook.