The document discusses different models of social sharing and networking that are centered around objects like photos, videos, bookmarks, and news stories. It covers first generation social networks like Friendster that connected people and second generation networks that put objects at the center. Specific models discussed include watercooler conversations around shared objects, viral sharing of interesting items, tag-based sharing linked by concepts, and social news creation through rating stories. The document also discusses challenges in designing social systems and balancing individual and social aspects.