An anonymous reader writes: According to a report from ProPublica, the world’s largest social network knows far more about its users than just what they do online. What Facebook can’t glean from a user’s activity, it’s getting from third-party data brokers. ProPublica found the social network is purchasing additional information including personal income, where a person eats out and how many credit cards they keep. That data all comes separate from the unique identifiers that Facebook generates for its users based on interests and online behavior. A separate investigation by ProPublica in which the publication asked users to report categories of interest Facebook assigned to them generated more than 52,000 attributes. The data Facebook pays for from other brokers to round out user profiles isn’t disclosed by the company beyond a note that it gets information “from a few different sources.” Those sources, according to ProPublica, come from commercial data brokers who have access to information about people that isn’t linked directly to online behavior. The social network doesn’t disclose those sources because the information isn’t collected by Facebook and is publicly available. Facebook does provide a page in its help center that details how to get removed from the lists held by third-party data brokers. However, the process isn’t particularly easy. In the case of the Oracle-owned Datalogix, users who want off the list have to send a written request and a copy of a government-issued identification in the mail to Oracle’s chief privacy officer. Another data collecting service, Acxiom, requires users provide the last four digits of their social security number to see the information the company has gathered about them.