Detecting selfish nodes in opportunistic mobile networks can reduce the loss of network resources, thus improve the data delivery performance. Most of existing detection schemes primarily rely on the nodes' contact records and do not consider their individual and social preferences in their data relaying behavior, which result in long detection time and high communication overhead. In addition, they cannot distinguish the nodes' selfishness type and degree, which is important because the charge and rewarding mechanisms applied to stimulate different nodes may not be the same. In this paper, we propose a Social-based Watchdog system (SoWatch) in which watchdog nodes analyze messages received from their encountered nodes with respect to their social tie information to identify the nodes' selfish behavior in message relaying. Meanwhile, the watchdog nodes apply the second-hand watchdog information received from other nodes to improve the detection time and accuracy. Next, we design a reputation system in which watchdog nodes identify selfish nodes based on their direct and indirect watchdog information and distinguish individually and socially selfish nodes. Furthermore, we design a watchdog evaluation module to protect SoWatch against wrong watchdogs disseminated by malicious nodes in which a watchdog node investigates the truthfulness of the indirect watchdogs before applying them. Our experiments using real-world datasets illustrate that SoWatch outperforms a benchmark contact-based watchdog system in terms of detection time by 45% and detection ratio by 10% with less communication overhead.