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.
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...
How is going to a 3rd party site like Google Reader (yes, it's dead and damn Google for killing it) any less discoverable than going to WaPo or Facebook? I make a conscience effort to go consume these sites. I made a conscience decision to consume my information from Google Reader. Or it's replacement.
RSS buttons are (were?) right along side Facebook, Twitter, Google+ buttons 10 years ago. Not that difficult.
If Facebook suddenly announced an RSS component to their service, people would use it. I know when Google reader dropped their Google Reader service, there was a hue and cry about it. Google said the usage numbers were flat. I suspect in great part because Google's lessor products come and go all the damn time. If the product is not integrated into Gmail or YouTube, 99.9% of their user base does not know of the service's existence.