We use a previously untapped data source, available via the Facebook’s advertising platform, to show the feasibility of nowcasting migration with Facebook data. Facebook allows advertisers – and researchers alike – to target Facebook users with certain characteristics, such as a particular age, gender, country of origin and of residence, education level, or topical interest. Before an ad is launched, and before any advertising cost is incurred, the platform provides an estimate of how many users match the provided criteria. This functionality, which can be accessed in a programmatic way and in real-time, allowed us to produce estimates of stocks of international migrants that closely align with those from the United Nations and the American Community Survey. Facebook data, that can be thought of as a regularly updated digital census across roughly 1.7 billion users present opportunities and challenges for demographic and survey research that we discuss in this article.