you have a single sign on that allows you to create multiple persona's e.g. public, family and friends
when posting you would have the ability/requirement to tag content as being accessible to individuals in groups you have associated with each persona.
sounds like a bit of a pain in the ass that would become very unwieldy beyond a few groups.
Not only for facebook, but the same applies to Google and other trackers. Many sites link to your Google account. This is possible because your cookies can be shared/accessible by scripts communicating between sites (and checking referers).
To isolate them you'd essentially need something like firefox profiles -- isolating cookies and credentials per persona. But to be more usable it needs to be more integrated into the browser somehow, and more lightweight, as running multiple firefox profiles is a bit clunky.