Facebook will "die" when it is no longer unique. They have defined the template for how all future websites need to operate. And I guarantee within a few years developers will find ways to standardize (or at least ubiquitize) every important feature of Facebook.

Once every website has a universal login, contacts, and sharing features, then why would I use Facebook, when I can use X, Y, or Z which have lots of other features too? And then the cycle will start over again.