What we need is a browser "persona" plugin, that allows you to swap in and out different accounts when it indicates you are on a FB enabled page.
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.