  In this work we analyze traces of mobility and co-location among a group of
nearly 1000 closely interacting individuals. We attempt to reconstruct the
Facebook friendship graph, Facebook interaction network, as well as call and
SMS networks from longitudinal records of person-to-person offline proximity.
We find subtle, yet observable behavioral differences between pairs of people
who communicate using each of the different channels and we show that the
signal of friendship is strong enough to stand out from the noise of random and
schedule-driven offline interactions between familiar strangers. Our study also
provides an overview of methods for link inference based on offline behavior
and proposes new features to improve the performance of the prediction task.
