One of the defining features of Web 2.0 is user-uploaded content, specifically photos. I believe that photo-sharing has quietly been the killer application which has driven the mass adoption of social networks. Facebook alone hosts over 40 billion photos, over 200 per user, and receives over 25 million new photos each day. Hosting such a huge number of photos is an interesting engineering challenge. The dominant paradigm which has emerged is to host the main website from one server which handles user log-in and navigation, and host the images on separate special-purpose photo servers, usually on an external content-delivery network. The advantage is that the photo server is freed from maintaining any state. It simply serves its photos to any requester who knows the photo’s URL.