Private, friends|family, do not share, etc. It’s very useful to be able to flag certain data you store as private; links or photos, event attendance, or notes. These simple defaults allow us to manage our data without the complex overhead of maintaining ACLs (a task almost no one is obsessive enough to manage)
But it would be nice if there was a way to re-define privacy on these sites so as to enable sharing among partners without the single account/password swapping solutions. Looking at the statistics I have to think this is not a niche feature.