—We study large online social footprints by collecting data on 13,990 active users. After parsing data from 10 of the 15 most popular social networking sites, we find that a user with one social network reveals an average of 4.3 personal information fields. For users with over 8 social networks, this average increases to 8.25 fields. We also investigate the ease by which an attacker can reconstruct a person’s social network profile. Over 40% of an individual’s social footprint can be reconstructed by using a single pseudonym (assuming the attacker guesses the most popular pseudonym), and an attacker can reconstruct 10% to 35% of an individual’s social footprint by using the person’s name. We also perform an initial investigation of matching profiles using public information in a person’s profile.