Another issue is that many online services today require us to share private information (think of Facebook applications), but beyond record-level access control we do not understand what it means to share data, how the shared data can be linked, and how to give users fine-grained control over this sharing in an intuitive, but effective way. In addition, real data are not static but get larger and change over time; none of the prevailing techniques results in any useful content being re-leased in this scenario.