I prototyped this feature last year as an intern at Mozilla. I think the engineering done for containers can be exposed in a lot of cool ways to the user. I'd love for you to send in your ideas on the comment form [1]. One of the features we considered last summer was "site specific containers" [2], which would support your facebook example.
Is it possible to implement this feature via the extension API ? Exposing the low-levels, like choosing the cookie jar or intercepting calls to the local storage, would sprawl some really good privacy-enhancing extensions.
EDIT: Looks like there are already extensions doing that ! nice !
EDIT: Looks like there are already extensions doing that ! nice !