Forwarding in Pocket Switched Networks, which make use of human mobility and local forwarding to deliver messages, is a challenging problem since human mobility is difficult to predict. Social information can be used for making forwarding decisions, for it varies much slowly than the topology of the network. Previous social based forwarding schemes exploit social centrality and community to make forwarding decisions. In this paper, we seek to make use of more relay candidates with relatively low centrality. Based on this intuition, we propose an improved social based forwarding algorithm based on BUBBLE scheme, called BiBUBBLE, which exploit extra social relation information besides centrality and community. The simulation results demonstrate that our algorithm improves the delivery rate with moderate delivery cost comparing to previous social based routing schemes.