People spend hours a day interacting in online settings, ranging from social media sites to a broad range of digital communities designed for work, education and entertainment. Such systems are generally intended to elicit particular activities or forms of engagement, yet we have relatively little understanding of the resulting behaviors or of how system design may contribute to those behaviors. In this talk, I will discuss some of our work that aims to develop models of human behavior in online settings, both to inform system design but also to address fundamental questions in the social sciences. In particular, I will discuss some results regarding the diffusion of ideas and products, participation in massive online courses, the use of badges as incentives, the inadvertent disclosure of social ties, and effects of homophily, or “birds of a feather” principle, in online communities. These results include the study of large sites such as Coursera, Flickr, LinkedIn, and Stack Overflow.