What is the effect of (1) popular individuals, and (2) community structures on the retransmission of socially contagious behavior? We examine a community of Twitter users over a five month period, operationalizing social contagion as ‘retweeting’, and social structure as the count of subgraphs (small patterns of ties and nodes) between users in the follower/following network.We find that popular individuals act as ‘inefficient hubs’ for social contagion: they have limited attention, are overloaded with inputs, and therefore display limited responsiveness to viral messages. We argue this contradicts the ‘law of the few’ and ‘influentials hypothesis’.We find that community structures, particularly reciprocal ties and certain triadic structures, substantially increase social contagion. This contradicts the theory that communities display lower internal contagion because of the inherent redundancy and lack of novelty of messages within a community. Instead, we speculate that the reasons community structures show increased social contagion are, first, that members of communities have higher similarity (reflecting shared interests and characteristics, increasing the relevance of messages), and second, that communities amplify the social bonding effect of retransmitted messages.