I've never worked in adtech (and hopefully never will), but the first idea that would pop into my head for trying to track people who sequestered their cookies like this would be to have the sites who put in the third-party cookies include the IP address. If only one Facebook account is known to log into that IP, you know who it belongs to, even when there's no active login.

I imagine that people actively trying to make money off stuff like this would have come up with plenty more ideas than just this one.