Pervasive sensor networks are the source of continuous data streams about our physical environment. With the rise of the Mobile Web people-centric sensing yields a new layer of spatiotemporal contextual data, from qualitative user-generated content (e.g. geo-referenced multimedia messages) to quantitative sensor measurements (e.g. earthquake or hazard alerts). This mobile sensed content is made accessible within an ecosystem of heterogenous service providers, from social networks to social data networks. The mining, analysis and processing of these streams provides many challenges and semantic technologies can be utilized to overcome this heterogeneity. The integration of sensor data with a user-generated context will provide an increased situational awareness and contextual knowledge, resulting in application scenarios from more efficient emergency response management to improved urban planning.