[DjangoReinhardt]

It's called shadow profiling[0].

Even if you haven't signed up for a facebook account but someone you know has an account, they by proxy, you have an account. Merely existing as a contact in their address book is enough to create a shadow profile with facebook. Their deep-learning algos can collate such data from all your friends and serve you correspondingly appropriate ads.

In fact, I just thought of an experiment that you could try sometime.

1. Buy a new cellphone number and store a small number of contacts (say, about 5) in your phonebook. Make sure all 5 of them have functional & active FB accounts.

2. Install the FB app on the phone and grant it access to your phonebook.

3. Open the app, create a new user and check out the list of suggested friends.

I think you'll be surprised.

[0] http://www.dailydot.com/news/facebook-shadow-profiles-privac...