  Despite global connectivity, societies seem to be increasingly polarized and
fragmented. This phenomenon is rooted in the underlying complex structure and
dynamics of social systems. Far from homogeneously mixing or adopting
conforming views, individuals self-organize into groups at multiple scales,
ranging from families up to cities and cultures. In this paper, we study the
fragmented structure of the American society using mobility and communication
networks obtained from geo-located social media data. We find self-organized
patches with clear geographical borders that are consistent between physical
and virtual spaces. The patches have multi-scale structure ranging from parts
of a city up to the entire nation. Their significance is reflected in distinct
patterns of collective interests and conversations. Finally, we explain the
patch emergence by a model of network growth that combines mechanisms of
geographical distance gravity, preferential attachment, and spatial growth. Our
observations are consistent with the emergence of social groups whose separated
association and communication reinforce distinct identities. Rather than
eliminating borders, the virtual space reproduces them as people mirror their
offline lives online. Understanding the mechanisms driving the emergence of
fragmentation in hyper-connected social systems is imperative in the age of the
Internet and globalization.
