[ycmbntrthrwaway]

You can scale it by classifying fields into groups. Then user can decide gradually if he wants to share his name. Later, application may ask for location (browsers can already do this), employer info, contact data etc.

What is needed is integration with permission granting mechanisms, which already exist in browsers.

[itissid]

Actually this problem looks is a lot like how Android App permissions are managed. Not long ago some apps used to ask you at install time that they want access to your google email, contacts, wifi and so on. A better app permission model (which I think is now in android) is to ask these "lazily" as the app needs them and let the user decide at that point.