Slides from my talk at the IEEE Conference on Visual Analytics Science and Technology (VAST) 2014 in Paris, France. ABSTRACT Logging user activities is essential to data analysis for internet products and services. Twitter has built a unified logging infrastructure that captures user activities across all clients it owns, making it one of the largest datasets in the organization. This paper describes challenges and opportunities in applying information visualization to log analysis at this massive scale, and shows how various visualization techniques can be adapted to help data scientists extract insights. In particular, we focus on two scenarios:\ (1) monitoring and exploring a large collection of log events, and (2) performing visual funnel analysis on log data with tens of thousands of event types. Two interactive visualizations were developed for these purposes: we discuss design choices and the implementation of these systems, along with case studies of how they are being used in day-to-day operations at Twitter.