  We describe a general stochastic processes-based approach to modeling
user-contributory web sites, where users create, rate and share content. These
models describe aggregate measures of activity and how they arise from simple
models of individual users. This approach provides a tractable method to
understand user activity on the web site and how this activity depends on web
site design choices, especially the choice of what information about other
users' behaviors is shown to each user. We illustrate this modeling approach in
the context of user-created content on the news rating site Digg.
