When people upload their contacts to Facebook to "find friends by phone or email" Facebook takes the phone numbers and email addresses of people who do not have Facebook accounts and associates them with a shadow profile. If multiple people upload contacts and have this same phone number and email address of a person, they can associate any other data uploaded (obviously name, birthday, address, other social media profile URLs?) with the same shadow profile. When the person creates an account using an email address or phone number tied to the shadow account, they already have a reasonable amount of data on the peson, including every user who has them in their contacts and others on those social graphs so that they probably know what school the user went to and current/past employers.
Is it kind of weird to call a shadow a shadow when it's just connecting the dots from yourself to the ground? What do you mean it's weird to call it a shadow profile? Just because you know how they do it does not make it "in the clear" or something not-shadow-like.
A shadow profile is a profile unknown to the victim and likely unknown to their friends. Facebook creates huge datasets of information about people without those people ever signing up for Facebook or even hearing about it. That's a shadow profile and it's creepy.