I'm maintaining a list of early adopter in the JSON-LD Wiki
If you want a more visual representation, you might wanna look at http://slideshare.net/lanthaler/building-next-generation-web-ap-is-with-jsonld-and-hydra/34
Google supports JSON-LD for embedding structured data in emails.
From their developer documentation:
Gmail, Search Answer Cards, and Google Now rely on structured data in emails to work. Schemas in Gmail supports both JSON-LD and Microdata and you can use either of them to markup information in email. This lets Google understand the fields and provide the user with relevant search results, actions, and cards. For example, if the email is about an event reservation, you might want to annotate the start time, venue, number of tickets, and all other information that defines the reservation.
The Digital Public Library of America uses JSON-LD
This site used JSON-LD to markup their recipes and successfully got rich snippets in Google:
A very important move forward in terms of JSON-LD annotated web.
Our team is working on the idea of the Semantic Web based on JSON-LD. Since you are interested in this format, you may be interested in learning about the project as well.
WRIO Internet OS — All You Need is a Browser (Google doc)
I really don't know what your beef is with this technology, or what it is you think you are critiquing, but it's clear you have something in your head that you think defines a technology. The use of various linked data approaches has been accepted by such "small" companies as Google, Facebook, Microsoft and Yahoo, and is becoming a mainstay of a number of projects including, but not limited to, Open Data. If there's a specific criticism that is motivating your question, then I'd suggest doing some homework, understanding what it is you are asking, and being more precise.
There's a lot of informed and spirited debate that took place around Project Open Data - https://github.com/project-open-data/project-open-data.github.io/pull/21.
[Disclaimer - I am a Sr. API Strategist at GSA]
Current statistics: as of January 2019, JSON-LD is used by ~23% of all the websites.
I suppose that almost all those sites use JSON-LD for SEO-related purposes, and about half of them uses JSON-LD more or less unknowingly (content management systems just generates stub JSON-LD snippets).