In fact, deactivating/"quitting" your FB account is not enough, either.
I deactivated my FB account several weeks ago, not so much for privacy issues directly but out of concern of the overall psychological effect of so much sharing and the emphasis on superficial identity (something I don't see discussed much).
Anyway, I checked my cookies after reading this piece and, not surprisingly, FB didn't remove my old auth cookie (the one keyed 'datr') when I quit their site. I should have known better, but I still think it's shameful to some degree to track people after they've very clearly disengaged from the site and their FB "identity".
datr is not an auth cookie. xs is an auth cookie, and c_user is the cookie which stores your user ID. datr is a machine identifier, not a user identifier (multiple people using the same computer share a datr, but don't share accounts, while a single user using multiple computers or browsers will have a single account, but one datr per machine).