With the massive prevalence of smartphones, mobile social sensing systems in which humans acting as social sensors respond to geo-located crowdsourcing tasks, became extremely popular. Such systems can provide significant benefits particularly during crisis management and emergency situations. However, not only querying users can be extremely costly but also human sensors are mobile, subjective and their response delays can highly vary. In this paper we develop a social sensing system that performs sampling on mobile social sensors to achieve accurate and real-time detection of the state of emergency events. Our contributions are two-fold: (i) our approach can capture well emergencies even in large geographical regions, and (ii) our sampling approach considers the individual characteristics of the social sensors to maximize the probability of receiving accurate responses in a timely manner. We provide comprehensive experiments that indicate that our approach accurately identifies critical real-world events, has low overhead and reduces the classification error up to 90% compared to traditional approaches.