http://worrydream.com/TheWebOfAlexandria/
Most URLs disappear within a few months. On the other hand, it's extremely hard to delete your private data (because of Facebook, Google, etc.).
So the data that you want will disappear and the data that you don't want will stay.
It's similar to APIs. It's extremely hard to exchange the data that you want between apps but your private data has no issue being exchange between Twitter - Facebook - Google etc.