We propose a trust-threshold based routing protocol for delay tolerant networks, leveraging two trust thresholds for accepting recommendations and for selecting the next message carrier for message forwarding. We show that there exist optimal trust threshold values under which trust-threshold based routing performs the best in terms of message delivery ratio, message delay and message overhead. By means of a probability model, we perform a comparative analysis of trust-threshold based routing against epidemic, social-trust-based and QoS-trust-based routing. Our results demonstrate that trust-threshold based routing operating under proper trust thresholds can effectively trade off message delay and message overhead for a significant gain in message delivery ratio. Moreover, our analysis helps identify the optimal weight setting to best balance the effect of social vs. QoS trust metrics to maximize the message delivery ratio without compromising message delay and/or message overhead requirements.