Participation to challenges within social networks is a very effective instrument for promoting a brand or event. In this paper, we take the challenge organizer’s perspective, and we study how to raise the engagement of players in challenges where the players are stimulated to create and evaluate content, thereby indirectly raising the awareness about the brand or event itself. We illustrate a comprehensive model of the actions and strategies that can be exploited for progressively boosting the social engagement during the challenge evolution. The model studies the organizer-driven management of interactions among players, and evaluates the effectiveness of each action in light of several other factors (time, repetition, third party actions, interplay between different social networks, and so on). We evaluate the model through a set of experiment upon a real case, the YourExpo2015 challenge. Overall, our experiments lasted 9 weeks and mobilized hundreds of thousands of users on two different social platforms; our quantitative analysis assesses the validity of the model.