Investigating sources of PII used in Facebook’s targeted . . . In this paper, we focus on Facebook and investigate the sources of PII used for its PII-based targeted adver-tising feature We develop a novel technique that uses Facebook’s advertiser interface to check whether a given piece of PII can be used to target some Facebook user, and use this technique to study how Facebook’s adver-tising service obtains users’ PII We investigate a range of