  Social networks amplify inequalities due to fundamental mechanisms of social
tie formation such as homophily and triadic closure. These forces sharpen
social segregation reflected in network fragmentation. Yet, little is known
about what structural factors facilitate fragmentation. In this paper we use
big data from a widely-used online social network to demonstrate that there is
a significant relationship between social network fragmentation and income
inequality in cities and towns. We find that the organization of the physical
urban space has a stronger relationship with fragmentation than unequal access
to education, political segregation, or the presence of ethnic and religious
minorities. Fragmentation of social networks is significantly higher in towns
in which residential neighborhoods are divided by physical barriers such as
rivers and railroads and are relatively distant from the center of town. Towns
in which amenities are spatially concentrated are also typically more socially
segregated. These relationships suggest how urban planning may be a useful
point of intervention to mitigate inequalities in the long run.
