That may be true (e.g. Facebook knows my musical tastes as of 12 years ago, which aren't relevant today), but I might be willing to pay that price in exchange f

That may be true (e.g. Facebook knows my musical tastes as of 12 years ago, which aren't relevant today), but I might be willing to pay that price in exchange for transience. A social network that resisted bulk archiving by third parties, and removed content in a permanent and legally-binding way after a given interval, would be providing a genuinely useful service. When you put something up on the regular web, its lifetime is completely out of your control. Sure, there would be screenshots and individual copying, but such a service could prevent bulk mirroring.