re: "The email address I use for Facebook was created only for Facebook. I have never given it out to anyone else, ever. The inbox for that account only contains emails from Facebook."

Per the book "Chaos Monkeys", as well as others sources I would presume since, that single signal is not a "unique key". There are plenty of other ways being used to tie you to your devices (plural! as in, you're a known / constant as you move from phone to laptop to tablet, and so on.)

Mind you, neither is new, but Dragnet Nation and Chaos Monkeys are both insightful, and if you take your privacy and liberty serious frightening.

https://www.antoniogarciamartinez.com/chaos-monkeys/

http://juliaangwin.com/dragnet-nation-available-now/

Of the two, CM is the written to be more entertaining.