So, I deactivated my account maybe 6 months ago, and uninstalled the app long ago. Since then, I moved halfway across the country and, using a brand new laptop, a fake name and number, and a throwaway email address, created another profile so I could use their API.
People You May Know still had old high school friends, my old real estate broker (??), and someone I starred on GitHub. I have absolutely no idea how they connected that account to my old one, considering Google Mail is the only other service I've used on that laptop.
If you're not using a plugin such as Facebook Disconnect, pages that have a "Like this on Facebook" embed or similar can accidentally or deliberately reassociate you with your prior identity. Consider this scenario:
3) When your browser requests the button from Facebook, the referrer is "http://example.com/?user=3834" which is a URL that you visited a lot when your old Facebook login was active, and it was never visited by any Facebook users apart from you.
There are other similar ways they could link you to an old identity if they wanted to, some not necessarily blockable by these plugins, but the above would be simplest.
You either logged your new account into a mobile device and it pulled your contacts, or you gave facebook your phone number for account recovery / two-factor and they already had your contacts either from your old account, or if you use whatsapp or instagram.
You can't provide a fake phone number for account recovery / two-factor authentication, as they verify that you control the phone number before accepting it.
Fully block all the Facebook network, they have trackers on virtually everything. From ad network (including "free" apps) to all the sites/services in their network (WhatsApp, Instagram)
I'm not, in fact I'm using a different ISP entirely. You really think my behavior patterns are uniquely identifiable at FB's scale? I can't imagine how many other users must have similar typing/clicking/resting/etc. patterns.
Interestingly, my comment's getting down-voted without refutation. C'mon, let's talk :)
Anyway, no, I don't know if FB can do that at scale.
What I do know is that sites, especially complex ones like FB, like to track user interaction to evaluate UX (hover targets, click targets, time it takes to find call-out etc).
If I were a data science type at FB, and knew FB was collecting that stuff, I think I'd like to find out what other questions I could answer with it.
Or more banal - did you use your laptop as a wifi endpoint and connect your phone & WhatsApp to it?
People You May Know still had old high school friends, my old real estate broker (??), and someone I starred on GitHub. I have absolutely no idea how they connected that account to my old one, considering Google Mail is the only other service I've used on that laptop.