Social ties form the bedrock of the global economy and international political order. Understanding the nature of these ties is thus a focus of social science research in fields including economics, sociology, political science, geography, and demography. Yet prior empirical studies have been constrained by a lack of granular data on the interconnections between individuals; most existing work instead uses indirect proxies for international ties such as levels of international trade or air passenger data. In this study, using several billion domestic and international Facebook friendships, we explore in detail the relationship between international social ties and human mobility. Our findings suggest that long-term migration accounts for roughly 83% of international ties on Facebook. Migrants play a critical role in bridging international social networks.