I still hold massive butthurt towards Google for abandoning Reader. It marked a pivot away from open technology platforms. Perhaps people don't know about RSS because Google / FB / Twitter prefer you not use it? These giants encourage passive consumption and discourage DIY curation of content. If I can currate my own content lists, I bypass their ad revenue generating framework, picking and choosing only those things I want to read. They receive no information about why I choose that specific article to read. I've made it a priority to trim my usage of Google to the extent possible. I dream of someday abandoning Gmail.
Good article from back in the day about the whys and wherefores of Google Reader's demise.
https://gigaom.com/2013/07/03/google-readers-shutdown-the-ri...
Facebook is discoverable because people invite you there, sites link to it, and you're often prompted to "log in with your Facebook account" into every damn thing you visit. People talk about Facebook; you hear it on the news, etc.
Facebook per se isn't discoverable. If Zuck had just registered "facebook.com", the domain, and put a server there and waited without promoting it, there would be no Facebook.