Engaging consumers through social media is a successful way for firms to get users attention and allow them to participate in content creation by two-way communication. However, social network sites (SNS) also challenges brands as users have the ability to change the narrative expressed by a firm with non-favorable content. This paper presents a process-oriented model to study one of the most frequent types of negative word of mouth: complaint behaviors. Given the collaborative and social characteristics of SNS, and drawing on literature in social psychology and consumer behavior, we theorize that cognitive aspects (e.g. collective efficacy) largely mediate the effects of dispositional factors (perceived utility) on complaining. A survey to a nationally representative sample of online Chileans show that even after controlling for factors recognized by previous research able to influence negative word of mouth (e.g., trust in companies, altruism or exposition to complaints on SNS), the level of collective efficacy affects consumers’ willingness to complain in SNS and relate to others their experiences with brands, services or products.