Thanks to widely available, cheap Internet access and the ubiquity of smartphones, millions of people around the world now use online location-based social networking services. Understanding the structural properties of these systems and their dependence upon users' habits and mobility has many potential applications, including resource recommendation and link prediction. Here, we construct and characterise social and place-focused graphs by using longitudinal information about declared social relationships and about users' visits to physical places collected from a popular online location-based social service. We show that although the social and place-focused graphs are constructed from the same data set, they have quite different structural properties. We find that the social and location-focused graphs have different global and meso-scale structure, and in particular that social and place-focused communities have negligible overlap. Consequently, group inference based on community detection performed on the social graph alone fails to isolate placefocused groups, even though these do exist in the network. By studying the evolution of tie structure within communities, we show that the time period over which location data are aggregated has a substantial impact on the stability of place-focused communities, and that information about placebased groups may be more useful for user-centric applications than that obtained from the analysis of social communities alone.Networks can describe a large variety of complex systems, and network science has proved to be a successful framework for the quantitative study of their structure and dynamics [1][2][3]. In the last decade, the tools and models provided by complex network theory have enabled discovery of similarities between seemingly very different systems including the Internet, the human proteome, and collaboration networks. Complex networks analysis is now regularly employed to characterise the topology and functioning of biological, technological and social structures [4,5].The analysis of social networks is one of the traditional application fields of network science, and sociologists generally agree that many social behaviours, from opinion formation to rule enforcement, from individual success to cooperation, depend in a fundamental way on the structure and evolution of the patterns of social relationships. In other words, characterising and quantifying social structures is often a prerequisite for understanding and interpreting social dynamics [6][7][8]. In the last twenty years, sociologists have relied on the study of small social networks with tens or hundreds of nodes at most, collected by means of targeted questionnaires and direct interviews. Recently, the ubiquity of the Internet and the World Wide Web, and the emergence of hundreds of online social services, have produced a huge volume of data about online relationships between millions of people around the world. These online social networks (OSNs) have allowed quantitative verification of sociologi...