Social behaviors emerge from the exchange of information among individuals—constrained by and reciprocally influencing the structure of information flows. The Internet radically transformed communication by democratizing broadcast capabilities and enabling easy and borderless formation of new acquaintances. However, actual information flows are heteroge- neous and confined to self-organized echo-chambers. Of central importance to the future of society is understanding how existing physical segregation affects online social fragmentation. Here we show that the virtual space is a reflection of the geographical space where physical interactions and proximity-based social learning are the main transmitters of ideas. We show that online interactions are segregated by income just as physical interactions are, and that physical separation reflects polarized behaviors beyond culture or politics. Our analysis is consistent with theoretical concepts suggesting polarization is associated to social exposure that reinforces within group homogenization and between group differentiation, and they together promote social fragmentation in mirrored physical and virtual spaces.