The short answer is it depends. On a basic website with a handful of pages, breadcrumbs are certainly unnecessary. But on larger sites (especially reference and documentation sites), breadcrumbs are extremely useful for navigation and orientation—one might even say they're downright necessary. The hard part is determining whether your site is large enough to warrant breadcrumbs or not.

All of the major players that you referenced do in fact use breadcrumbs on their sites; it's just that they only use them in certain contexts:

StackExchange Help Center

http://ux.stackexchange.com/help/answer-banshttps://ux.stackexchange.com/help/answer-bans

Facebook Developer Reference

https://developers.facebook.com/products/social-plugins/like-share-send/

Google API Reference

https://developers.google.com/maps/documentation/javascript/reference

YouTube API Reference

https://developers.google.com/youtube/v3/code_samples/apps-script