This thesis is a set of three studies concentrated on the antecedents and effects of users trust in online health groups on Facebook. People in Brazil are increasingly using these groups as a way to find health information and support from people who experience the same health challenges. There are several types of online groups addressing various topics of interest gathering together hundreds and thousands of individuals who can easily participate and interact with others. Their goal is to be able to improve their knowledge about their topic of interest, to manage their health condition better. Also many seek advice to better deal with the disease and alleviate its symptoms. Others are seeking support for behavior change to be healthier. The participation of people in such groups has the potential to address many health issues and positively influences health systems. So it is important to better understand about aspects that can contribute to the online group's sustainability and longevity, such as the activity of members, its inner workings and the relationship between members. These online groups are constituted by people who do not know each other personally. Thus, trust is an aspect that materializes from the interactions experienced and relationships constituted within the community. These experiences, result of the interaction of the individual with the other participants and the knowledge available, has the potential to increase or decrease their trust in the online groups. Which in turn impacts the way people engage in the community and contribute their knowledge as well as the adoption of knowledge available in the online group. The first study discusses aspects of the activity of members and the online group inner workings. These aspects imply the vitality of online communities, a crucial element for its success and development. Based on previous literature, we identified that there is no measure to assess the vitality of the community, although its dimensions have already been conceptualized. The main contribution of this paper is the development and validation of the Online Community Vitality scale, which can be measured by 20 items composed of five sub-dimensions: content quantity, content quality, interactivity, responsiveness, and atmosphere. The second study seeks to address the literature gap by exploring the determinants of individuals trust in online health groups. The main contribution of the second paper is the extension of the literature of trust in the context of social medias by testing and validating new variables as antecedents of trust: online community vitality, community support and perceived information credibility. The last study proposes that the participant's trust in online health groups have the potential to increase their engagement, knowledge adoption and contribution. Also, proposes that engagement positively influences the adoption of knowledge and knowledge contribution. The results support all prepositions. The findings contribute to the expand the literature about trust and engagement, considering the context of online health groups on Facebook.