These things share what is fashionable, not what is important. If I find an interesting, low volume blog, I can subscribe to its feed and I
will get an update when content is added. With your scenario, I
might get an update if content is added, if I can find it between guff about conference scandals. It's simply not comparable, unless you're forcing worthy content (in my judgement) to have the same value as transitory celebrity gossip.
These services give me a good chance of discovering things, RSS lets me follow them. As a case in point, there are a lot of things in my RSS feed that I have discovered from HN because one single post from them made it to the front page, but no subsequent one has. But I get to see the updates because I subscribed to their feeds. That subsequent ones haven;t made it to the HN front page is absolutely not a value judgement on their worthiness - unless, as I say, you think 5 same-but-different articles on who said what and to whom at a conference is more important than the infrequent technical article by a researcher.
In short, I want to follow the stuff on the right of the bell curve, not in the middle. The stuff that mostly floats to the top of what you're advocating (HN, fb, twitter) is firmly in the middle. A lot of the people posting the content I find interesting don't have twitter feeds announcing their posts - that kind of thing is often the preserve of this week's preferred javascript framework project. My RSS feeds allowed me a much higher SNR than any of these things. So please don't compare them, as if they can serve the same purpose. That's nonsense.