The drastic development of information and communication technologies (ICT) has given birth to many world-leading Internet enterprises such as Alibaba and Tencent. Various big geo-data generated by these internet giants provide unprecedented opportunities to understand the structures and observe the dynamics of Chinese cities from the perspective of human behavior. In this talk, the term social sensing is introduced as the theoretical framework of the analytical methods for multi-source big geo-data. These methods have been widely applied to sense different aspects and systems of cities. This talk will summarize the methods from two aspects: place semantics and spatial interactions. A number of case studies, at both intra-urban and inter-urban scales, are presented to substantiate the value of big geo-data.