People want full ownership and control of their information so they can turn off access to it at any time. At the same time, people also want to be able to bring the information others have shared with them—like email addresses, phone numbers, photos and so on—to other services and grant those services access to those people’s information. These two positions are at odds with each other. There is no system today that enables me to share my email address with you and then simultaneously lets me control who you share it with and also lets you control what services you share it with.
“On Facebook, People Own and Control Their Information”, Mark Zuckerberg, Facebook blog.