It is worth noting that the world’s biggest photo-sharing website is actually a social network: Facebook. In his history of the site, The Facebook Effect (2010), David Kirkpatrick describes how the photo-sharing feature was adopted by users:

“For every screenful of shots of girls there were only a few photos of guys. Girls were celebrating their friendships … Ordinary photos had become, in effect, more articulate. They conveyed a casual message. When it was tagged, a photo on Facebook expressed and elaborated on your friend relationships. “Pretty quickly we learned people were sharing these photos to basically say, ‘I consider these people part of my life, and I want to show everyone I’m close to them,'” says Sittig. Now there were two ways on Facebook to demonstrate how popular you were: how many friends you had, and how many times you had been tagged in photos … In Facebook, photos where no longer little amateur works of art, but rather a basic form of communication … A month after it had launched, 85 percent of the service’s users had been tagged in at least one photo.”