Taking that one step further, maybe if someone wrote that kind of tool, basically an API for your personal data that you can run locally, maybe they could scale

Taking that one step further, maybe if someone wrote that kind of tool, basically an API for your personal data that you can run locally, maybe they could scale it up to what you're describing? It would require a massive paradigm shift in how the internet works in practice, but image a world where: 1. You sign up for some new social network/thing that requires your data 2. You point it at your own personal (or hosted) info server 3. The service promises/or the use of the personal info server stipulates states that the data should never be stored longer than the lifetime of a relevant transaction 4. The social network queries your data maybe once a day (or more depending on whatever kind of work it does), announces itself, and can be cut off when you want. Of course, things don't sell these days with just privacy these days, but there's some upside to a service like this ironically due to it's centralization -- you could sell the user on "only enter your address/personal/credit card info once, have it available everywhere with one click, no long login forms"