i was wondering if this is more a problem with how third parties are handling SSL custom domains rather than HPKP. I would have thought SSL custom domains would have been you just upload a certificate/private key you want to use or you click a checkbox saying that you are happy to use letsencrypt. But I'm guessing for various reasons (no SNI support, etc) some of these companies are using a massive shared certificate.

also, if you are building a competing product with someone that is doing custom SSL domains it might be useful to check out if your competitor is using a shared cert because you can get a list of companies that are paying for the premium product :)