  Every day millions of users are connected through online social networks,
generating a rich trove of data that allows us to study the mechanisms behind
human interactions. Triadic closure has been treated as the major mechanism for
creating social links: if Alice follows Bob and Bob follows Charlie, Alice will
follow Charlie. Here we present an analysis of longitudinal micro-blogging
data, revealing a more nuanced view of the strategies employed by users when
expanding their social circles. While the network structure affects the spread
of information among users, the network is in turn shaped by this communication
activity. This suggests a link creation mechanism whereby Alice is more likely
to follow Charlie after seeing many messages by Charlie. We characterize users
with a set of parameters associated with different link creation strategies,
estimated by a Maximum-Likelihood approach. Triadic closure does have a strong
effect on link formation, but shortcuts based on traffic are another key factor
in interpreting network evolution. However, individual strategies for following
other users are highly heterogeneous. Link creation behaviors can be summarized
by classifying users in different categories with distinct structural and
behavioral characteristics. Users who are popular, active, and influential tend
to create traffic-based shortcuts, making the information diffusion process
more efficient in the network.
