>> Free Basics puts no obligation on its users to visit or use Facebook at all.
Here is a list of possible ways Facebook can triangulate information about people who are not active on Facebook even without controlling access to the internet:
1. User telemetry based on your browser when you visit any page which has a Like button
2. If you have ever used WhatsApp and deleted it, seeing who has your phone number on their contacts list
3. If you have ever used WhatsApp and deleted it, seeing who has your phone number on their WhatsApp app
4. Being tagged in a photo (that your friend captured) based on a Facebook account which you don't even actively use
5. Seeing where they tagged you if that info is easy to infer from the image's metadata
6. Seeing where they tagged you based on the check in location
7. Seeing where they tagged you, based on the comment they might leave on the photo
8. Seeing who has your email address on their contacts list and associated a name to it
9. Inferring your name from someone's contacts and matching it to the phone number so that
10. They can check if a second person has the same phone number or same email address under the same name or a small variant.
11. Learning about life events which concern you based on other people's WhatsApp conversations about you since they usually mention you by name
12. Text mining of WhatsApp conversations for a list of names which they don't yet have in their database and possibly inferring the relationship type
Now they basically have a "shadow profile" of a (person's name + phone number) even if that person has never gone near Facebook their entire lives. If there are additional details about you in someone else's contacts, imagine how much easier you just made it for Facebook.
12. If you agree that the shadow profile is fairly easy to create once the apps controlled by FB (FB, Messenger, WhatsApp, Instagram) reach a sufficient critical mass, you probably understand that they will collect every piece of information they can and associate it with the right shadow profile.
My guess is, I don't cover a fraction of the techniques they use already (simply because at a certain scale, I bet they identified even bigger patterns that we cannot even see), and the list is already too uncomfortable for someone who doesn't want to have anything to do with Facebook. As a programmer, you probably know that most of these are trivial to implement. Your problem will be "too much noise", definitely not "too little signal". No worries. An eager horde of people are everyday helping Facebook to cut down the noise and increase the signal to noise ratio of their shadow profiles.
Now, imagine what happens when Facebook becomes the primary means to access the internet for an entire region.