A small list of websites that install their custom fonts can cause you to picked out of a crowd even when other things (like your IP) change. The centralization of the internet probably helps a bit here, but I'm guessing that most people go to smaller websites at least occasionally rather than just spending all their time on a handful of social media sites.
Hm, what if each site install its own custom font to intentionally track users across websites?
You could generate a font per user and bam. Damn, is that really it?
Yeah, it sounds too easy, so wonder if I'm missing something. Weird how the site can even tell what fonts you have.