Given a social network, who are the key players controlling the bottlenecks of influence propagation if some persons would like to activate specific individuals? In this paper, we tackle the problem of selecting a set of k mediator nodes as the influential gateways whose existence determines the activation probabilities of targeted nodes from some given seed nodes. We formally define the k-Mediators problem. To have an effective and efficient solution, we propose a three-step greedy method by considering the probabilistic influence and the structural connectivity on the pathways from sources to targets. To the best of our knowledge, this is the first work to consider the k-Mediators problem in networks. Experiments on the DBLP co-authorship graph show the effectiveness and efficiency of the proposed method.