Social media mass adoption by individuals has increased consumer power, which, in turn, has pressured companies to adopt and manage its social media communications. However, there is limited research specifically on social media marketing adoption by companies. Furthermore, there is little consensus about the adoption factors and a lack of standardization in the terminology used in the academic articles. This thesis further the knowledge in social media adoption at the company level, by (i) identifying variables that can influence social media adoption by companies and determining how these variables influence adoption; (ii) proposing and empirically testing a theoretical model of social media adoption, under the light of TAM, UTAUT, and Institutional Theory. This thesis was conducted in two phases, which derived two articles. The first phase was primarily composed by analysis of the semi-structured interviews with senior marketing executives of large companies of different economic sectors. The second phase further developed the theoretical model and quantitatively tested it, based on the results of surveys sent to marketing professionals. The results of the second phase show that Isomorphic pressures (eg: coercive and mimetic pressures), social influence and facilitating conditions were the key factors driving social media adoption.