In DHH’s foreward, near the bottom of the first page, is this text:
Think of all the incredible applications that run the internet with Rails as
their backbone. Like Shopify, GitHub, Intercom, Procore, Cookpad, Intercom,
Fleetio, Instacart, Gusto, Zendesk, Coinbase, and many, many more.
See Intercom in that list twice.