The document discusses models of popularity and sharing on social networks and websites. It outlines 5 observations about social networks today, including the rise of second generation networks and object-mediated sociality. It then describes 3 models of popularity: watercooler conversations around shared objects, viral sharing of interesting content, and tag-based social sharing. The document concludes with 10 principles for designing websites and systems to encourage sharing, such as making them personally useful and useful, allowing different levels of participation, and adding elements of serendipity, independence, and play.