Previous delay-tolerant network (DTN) routing algorithms exploit either past encounter records or social network properties to derive a node's probability of delivering packets to their destinations. However, they only have a local view of the network, which limits the routing efficiency. Also, when two nodes meet, they have to exchange the delivery abilities to the destinations of all packets in the two nodes, which incurs high resource consumption. In this paper, we propose SMART, which utilizes a distributed social map for lightweight routing in delay-tolerant networks. In SMART, each node builds its own social map consisting of nodes it has met and their frequently encountered nodes in a distributed manner. Based on both encountering frequency and social closeness of the two linked nodes in the social map, we decide the weight of each link to reflect the packet delivery ability between the two nodes. The social map enables more accurate forwarder selection through a broader view. Moreover, nodes exchange much less information for social map update, which reduces resource consumption. Trace-driven experiments and tests on the GENI ORBIT testbed demonstrate the high efficiency of SMART in comparison to previous algorithms.