I read a question on Information Security about how Facebook could know recommendations of friends by just a phone number.
The answer to that question explicitly details that the more probable scenario is one in which the friends of the phone contact shared the information to Facebook.
So let's start there. Let's assume a phone number is in a contact list "123-4567"
At this point, before a friend syncs to Facebook. All this is a number, not a name, not an address.
You can state here that Facebook holds phone numbers (because Facebook users allow them to) to assist with the Suggested Friends / People you may know feature.
With you interacting with Facebook in anyway
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In terms of the registration specifically, since you never finished the process then the user you registered isn't validated. Why would Facebook store dirty data since 2006?
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