Ethnic politics scholars are increasingly convinced that (a) the political salience of ethnicity and (b) the correlation between ethno-linguistic fractionalization (ELF) and poor development are driven by the dense social networks shared by co-ethnics. By this argument, social networks allow ethnic parties to leverage inbuilt networks to share information and support collective action, while ethnically fragmented communities struggle to hold politicians accountable. This paper provides the first comprehensive empirical test of the assumption underlying this argument. Using seven months of telecommunications data from 9 million mobile subscribers in Zambia — which includes records of almost 2 billion calls and SMS messages — to measure social networks across an entire country, this paper finds that electoral constituencies with high ELF also have more fragmented social networks, especially in rural areas. It also finds other potential cleavages that have not achieved political salience (namely, religious identity and employment sector) are not correlated with network fragmentation, consistent with the idea that ethnicity achieves salience because it offers an organizational advantage not offered by other cleavages. Finally, it also finds that both voter knowledge and public goods are negatively correlated with network fragmentation, consistent with the network-proxy hypothesis.