Or are you using your real mobile number for 2-factor auth on Facebook?
You're probably also giving that out to businesses you have a relationship with. And most people don't have a second fake phone number to give out to avoid matching.
It's a bit like grocery store loyalty cards: you can give the grocery store bogus contact info when you sign up for one, but the first time you swipe a credit card to pay for your groceries, the store gets your name and a unique ID linked to you (the card #) that can be used to match your profile in (say) an Acxiom database.
You're probably also giving that out to businesses you have a relationship with. And most people don't have a second fake phone number to give out to avoid matching.
It's a bit like grocery store loyalty cards: you can give the grocery store bogus contact info when you sign up for one, but the first time you swipe a credit card to pay for your groceries, the store gets your name and a unique ID linked to you (the card #) that can be used to match your profile in (say) an Acxiom database.