In most routing algorithms of the previous work, the flooding method is used to obtain auxiliary information, which wastes network resources. Motivated by this, a distributed social distance routing algorithm was proposed. Firstly, the stability and regularity of contact between nodes were analyzed to determine the friend relationship. Then, the social-distance between nodes was constructed by the friend relationship. In addition, a table for recording the shortest social distance to other nodes was maintained by each node, and the minimum social distance was continually updated by exchanging and comparing the information in the table. The construction of social distance only needs to exchange information among friends instead of all nodes, which can greatly reduce the time of auxiliary-information exchange. Finally, when the packet was sent to the relay node with a smaller social distance to its destination node, the delivery ratio could be significantly improved. The experimental results demonstrate that, compared with the Probabilistic Routing Protocol using History of Encounters and Transitivity (PRoPHET) algorithm, the delivery ratio of the proposed algorithm is increased by about 3%, the data packet transmission delay is reduced by about 27%, and the auxiliary-information exchange times is reduced by about 63%. Compared with the routing based on Betweenness centrality and Similarity (SimBet) algorithm, the delivery ratio of the proposed algorithm is increased by about 11%, the data packet transmission delay basically equals, the auxiliary-information exchange times is reduced by about 63%. The social distance algorithm provides a theoretic support for large-scale mobile opportunistic networks, because of its better performance in scalability.