There's a fine line between what is considered a knowledge database and an invasion of privacy, and that line is likely to be determined by marketing. This week we wrote an article about Please Rob Me - a service that identifies Foursquare and Gowalla check-ins on Twitter and lets others know that a person is not home. While location-based services are often touted for their social and recommendation-based benefits, the realization that they can be used negatively have many questioning the responsibility of those groups that collect the data. In mid January Facebook founder Mark Zuckerberg stated that the age of privacy had come to an end and we responded that evolving preferences were not a valid justification of the elimination of privacy preferences . Nevertheless, between cookie tracking and browser identifiers like those shown in the EFF's Panopticlick and the fact that it only takes your zip code, gender and birthdate to identify you, it's hard to ensure to...