This paper presents work in progress within the FP7 EUfunded project SMART to develop a multimedia search engine over content and information stemming from the physical world, as derived through visual, acoustic and other sensors. Among the unique features of the search engine is its ability to respond to social queries, through integrating social networks with sensor networks. Motivated by this innovation, the paper presents and discusses the state-of-theart in participatory sensing and other technologies blending social and sensor networks.