Social capital—the strength of an individual’s social network and community—has been identified as a potential determinant of outcomes ranging from education to health, but there is very limited measurement of social capital worldwide. We construct and analyze several global measures of social capital, including economic connectedness—the extent to which individuals form friendships across socioeconomic lines—using data on 550 billion social connections between 1.7 billion Facebook users across nearly 200 countries and territories. We find that economic connectedness varies substantially both across and within countries. Economic connectedness is associated with higher intergenerational economic mobility, greater trust in others and in democratic institutions, and more pro-social norms. We also identify several factors—population density, language homophily, and inequality that together predict two-thirds of the cross-region variation in homophily by socioeconomic status.