I presume you're not aware of Facebook's research into eye tracking for VR. It goes a lot further than just identity. I believe their main interest is gauging emotional response for better targeted advertising.

If you look at the original FT article which covered the patents that caused the whole stir [1] (paywalled but I found it in the usual places) you'll see that their patents only cover using eye tracking to render media. A comment in FT's interview with Clegg discusses using eye tracking to see if an advertisement was looked at, like how JS is used to track whether an ad was viewed now. "Gauging emotional response" is really, really stretching it. Meta does enough garbage today, let's not foment conspiracy here.

[1]: https://www.ft.com/content/76d40aac-034e-4e0b-95eb-c5d34146f...