They address this: “ To help CMU measure results, Facebook shares a single statistic known as a weight value that doesn’t identify a person but helps correct f

They address this: “ To help CMU measure results, Facebook shares a single statistic known as a weight value that doesn’t identify a person but helps correct for any sample bias, adjusting for who responds to survey invitations. Making adjustments using weights ensures that the sample more accurately reflects the characteristics of the population represented in the data. The weight for a survey respondent in the sample can be thought of as the number of people in the population that they represent based on their age, gender and state of residence.”