People with facebook-apps on their phones essentially allows Facebook to get their full contact list. If person A, B and C all have the number of a person D who is not on facebook, it's enough to create a shadow profile. On the basis of A, B and C, their social statuses, their level of education, country, etc., it's possible to make pretty accurate guesses to determine the person D.
If person D some day decides to install, say, WhatsApp (or even if he had it before Facebook acquired it), it also has his text messages, and photos shared through said app. It can all be linked to that shadow profile with that phone number. If D at some point typed his email-address into WhatsApp, or signed up at some site that shares data with Facebook, it's also easily linked to that shadow profile.
If person D some day decides to install, say, WhatsApp (or even if he had it before Facebook acquired it), it also has his text messages, and photos shared through said app. It can all be linked to that shadow profile with that phone number. If D at some point typed his email-address into WhatsApp, or signed up at some site that shares data with Facebook, it's also easily linked to that shadow profile.