Use case Photos are inherently-social event markers. We take pictures to remember an occasion and - often - people who were present with us at that point. While most of the photographs are not looked at more than once or twice, the more popular ones become very important in our history. The same does not happen to other people on our photograph, despite our best efforts to share. Emailing photos is cumbersome and subscribing to Flickr streams requires all people to look at all photos just in case they are in it.