Using an iphone without a phone number?

I have a friend who was an international student, who used an old iphone during his 2 years here (USA). He didn't have a phone number, and would communicate with people whenever he was hooked up to wifi through facebook messenger.

This got me thinking. I always see people online say it's "literally impossible" to use the internet and be anonymous and not have your internet usage tracked to *you.*

However I fail to understand how this is the case. …

facebook facebook messenger international iphone messenger old people phone phone number privacy student thinking usa wifi