In my dissertation, I focus on estimating network effects in online social networks, using observational data. In the first chapter of my dissertation, coauthored with David Easley and Eleonora Patacchini, we analyze the effect of peer influence on item adoption decisions on GitHub. The second chapter of my dissertation focuses on gender disparities in contributions and social network formation patterns on GitHub. The third chapter of my dissertation studies peer effects on video-game playing decisions on Steam. In each of the chapters of my dissertation, I deal with social selection by incorporating a machine learning algorithm, popular in online recommendation systems, to predict individual preferences based on previous adoption decisions.