In this paper we study the community structure of endorsement networks, i.e., social networks in which a directed edge u → v is asserting an action of support from user u to user v. Examples include scenarios in which a user u is favoring a photo, liking a post, or following the microblog of user v. Starting from the hypothesis that the footprint of a community in an endorsement network is a bipartite directed clique from a set of followers to a set of leaders, we apply frequent itemset mining techniques to discover such bicliques. Our analysis of real networks discovers that an interesting phenomenon is taking place: the leaders of a community are endorsing each other forming a very dense nucleus. Categories and Subject Descriptors: H.4.3 [Information Systems Applications]: Communications Applications General Terms: Experimentation.