The knowledge of the urban mobility is a crucial aspect for city planners and administrators. The huge amount of geo-spatial data, generated by the combination of social media systems and the wide use of smart devices, is creating new challenges and opportunities to satisfy this thirst of knowledge. In this work, we explore how social media data can be used to infer knowledge about urban dynamics and mobility patterns in a urban area. Specifically, in order to highlight the main advantages, limitations, and open issues, we focus on mobility patterns by presenting a survey of the state of the art and a case-study based on the city of Barcelona.