Delay-Tolerant Networks are characterized by not having permanent end-to-end connections, often leading to intermittent connectivity, long and variable delays and high error rates. Mobile Social Networks are particular scenarios on which nodes are seen as individuals with inherent social habits, having wireless communication devices. In this paper a routing protocol based on the social property of Friendship is proposed. The protocol allows nodes to consider other nodes to be friends if they maintain contact frequently, regularly and in long-lasting sessions. The forwarding scheme consists in only delivering the message to nodes which are friends of the destination. To evaluate the performance of this protocol, the ONE Simulator was used. The Friendship Protocol performance was analyzed and compared to three other routing protocols while varying the network load. The simulation results showed that the Friendship Protocol could reach a high delivery rate and a very low overhead. A dynamic threshold version of the protocol was also proposed, on which the friendship threshold changes over time as it corresponds to a portion of the best friends. The results of this dynamic version were similar to the fixed threshold version, but with roughly half the overhead.