Citing employees at Hamburg Data Protection, Bloomberg claims that Facebook's cookies will still actively track your online activity even if you've cancelled your account. But it mostly just seems like tin hat paranoia.
According to the report, there's "suspicion" and over the way Facebook is using cookies. What that means exactly is unclear, as they don't elaborate any further aside from saying that the cookies can identify specific people. Facebook says they delete any user specific cookies, but leave some for security purposes, such as phishing.
Remaining cookies are used in "identifying spammers and phishers, detecting when somebody unauthorized is trying to access your account, helping you get back into your account if you get hacked," and blocking underage users from re-registering with a different birth date, Facebook said.
Should Facebook be doing this without people knowing? Probably not. But even if they are collecting data on you after you cancel your account, is it different from what any other website is doing? If these are supercookies, which are considerably harder to get rid of, then yeah, it's problematic. But sites will drop a cookie on your computer and track your data even if you're just visiting—regardless of whether or not you have an account.