Measuring and forecasting migration patterns, and how they change over time, has important implications for understanding broader population trends, for designing policy effectively and for allocating resources. However, data on migration and mobility are often lacking, and those that do exist are not available in a timely manner. Social media data offer new opportunities to provide more up-to-date demographic estimates and to complement more traditional data sources. Facebook, for example, can be thought of as a large digital census that is regularly updated. However, its users are not representative of the underlying population. This paper proposes a statistical framework to combine social media data with traditional survey data to produce timely `nowcasts of migrant stocks by state in the United States. The model incorporates bias adjustment of the Facebook data, and a pooled principal component time series approach, to account for correlations across age, time and space. We illustrate the results for migrants from Mexico, India and Germany, and show that the model outperforms alternatives that rely solely on either social media or survey data.
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