As social network services become more pervasive, social media advertising emerges as an attractive vehicle for augmenting advertising effectiveness. To leverage this new means of marketing, one must understand what engages SNS users in a favorable online behavior (i.e., overtly indicating personal interest in, or support for, the exposed message by clicking the Like or Share button in Facebook), thereby resulting in an effective advertising campaign. This research conceptualizes SNS ad effectiveness as a concept encompassing emotional appeal, informativeness and creativity that all have a potential to contribute to a positive online behavior. It empirically investigates the antecedents of positive user behavior for a SNS ad based on the theory of reasoned action, the social influence theory, and a persuasion theory. It proposes and tests a conceptual model of the formation of online user’s behavioral responses with regards to SNS advertising. The results of our empirical tests of the model reveal that informativeness and advertising creativity were key drivers of favorable behavioral responses to an SNS ad and that intention to engage in favorable user responses was positively associated with purchase intention. Based on these findings, the paper suggests further research directions and offers implications for harnessing the full potential of the new SNS advertising platform.