Auditing offline data brokers via Facebook's advertising platform

Data brokers such as Acxiom and Experian are in the business of collecting and selling data on people; the data they sell is commonly used to feed marketing as well as political campaigns. Despite the ongoing privacy debate, there is still very limited visibility into data collection by data brokers. Recently, however, online advertising services such as Facebook have begun to partner with data brokers-to add additional targeting features to their platform- providing avenues to gain insight into data broker information. In this paper, we leverage the Facebook advertising system-and their partnership with six data brokers across seven countries-in order to gain insight into the extent and accuracy of data collection by data brokers today. We find that a surprisingly large percentage of Facebook accounts (e.g., above 90% in the U.S.) are successfully linked to data broker information. Moreover, by running controlled ads to 183 crowdsourced U.S.-based volunteers, we find that at least 40% of data broker sourced user attributes are not at all accurate, that users can have widely varying fractions of inaccurate attributes, and that even important information such as financial information can have a high degree of inaccuracy. Overall, this paper provides the first fine-grained look into the extent and accuracy of data collection by offline data brokers, helping to inform the ongoing privacy debate.