In the online world, user engagement refers to the quality of the user experience that emphasizes the positive aspects of the interaction with technology and, in particular, the phenomena associated with wanting to use that technology longer and frequently. In this talk, I will give an introduction to user engagement, I will present its characteristics as identified in the research and analytic literature and I will discuss the challenges associated with measuring user engagement. I will reveal similarities and differences in user engagement depending on the considered web services that allow for a better representation of how users engage. I will describe recent data-driven approaches looking at user engagement through the development of new measures that capture engagement across different web services.