There is a classic census problem of how to provide reasonably detailed statistical and geographic information about a population while still protecting the identities of individuals. Consider this problem for a moment, and then turn it upside down:
How do you provide information publicly about individuals without exposing your database in its entirety to those who might mine it for unintended purposes?
Tom Owad just released an incredibly detailed howto for analysing the reading habits of Amazon.com wishlist users. Did you think this sort of profiling was only available to the likes of the NSA and FBI?
Think again: