You make explicit that all data that people enter, they enter for purposes of sharing. At the same time, you ban creating profiles with data that has not been explicitly shared. IMO:
- Make a telefone-book style listing, or searching for "all metalheads < 25 near Chicago" where people entered that into their profiles -> OK
- Tracking users on your site -> OK
- Tracking users on third party sites, and then aggregating this data, so you can see "people who searched for baby carrages" or "people who bought diapers with their credit card" -> not OK
- Having some kind of database where people could concievably look up what user tqi purchased, searched, what their political affiliation is (when not made public) -> not OK (unless you have extreme auditibility, four-eye principle, and so on)