Empirical social networks are characterized by a high degree of triadic closure (i.e. transitivity, clustering), whereby network neighbors of the same individual are also likely to be directly connected. It is unknown to what degree this results from dispositions to form such relationships (i.e. to close open triangles) per se or whether it reflects other processes, such as homophily and more opportunities for exposure. These are difficult to disentangle in many settings, but in social media not only can they be decomposed, but platforms frequently make decisions that can depend on these distinct processes. Here, using a field experiment on social media, we randomize the existing network structure that a user faces when followed by a target account that we control, and we examine whether they reciprocate this tie absent (never existent or removed), we are able to attribute this effect specifically to a small variation in the stimulus that indicates the presence (or absence) of a potential mutual follower. Theory suggests that triadic closure should be especially likely in open triads of strong ties, and we find evidence of larger effects when the subject has interacted more with the existing follower. These results indicate a substantial role for dispositions toward triadic closure, which platforms and others can choose to leverage in encouraging tie formation, with implications for network structure and the diffusion of information in online networks. I contextualize these results in the broader context of the value of long ties — or ties not part of closed triads.

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