Most consumption-oriented social media is "like an RSS reader". HN is a news aggregator with a forum bolted onto the bottom of the stories.
What is Facebook? It's an RSS feed of your friends and family's lives. Facebook has replaced Blogger et al, which primarily syndicated through actual RSS, because it was an accessible, simple way to a) create a feed and b) allow publishers to retain some control over their content (by e.g. blocking an undesirable consumer).
You had to have some technical competency to set up a blog and an RSS reader, and all Facebook did was fix that and bring the power of syndication to the RSS-incapable masses.
As far as I'm concerned, Twitter's value proposition is that it's supposed to be a continuous, temporary real-time vein on whatever is happening at that moment. It doesn't really do a good job of that filling that niche IMO. The only time I've had an impulse to use Twitter is during something like a live news event, and these days I think something like Snapchat does this better (though they're both infuriatingly difficult to access and navigate, decimating their practical utility).
In practice Twitter has regressed to the lowest common denominator of "microblogging". The thing that continues to give it power is its interactivity. Because the messages are so short and feel so quick, Twitter ensures that people will see our message, react, and that hits our feedback loops ASAP, demanding follow-on after follow-on.
One interpretation of today's Twitter is that it's just the unwashed masses clamoring for a fleeting glimpse of attention from someone they admire. This corresponds with Twitter's early adoption as a celebrity messaging platform for actors (iirc Ashton Kutcher was the first Twitter user to get 1M followers).
The only time Twitter's current incarnation is useful is when it allows the everyman to get a message to someone else who would be guarded, e.g., common citizens potentially inciting a credible presidential campaign as in the infamous Russ_Steinberg tweet [0] .
It's depressing to think that so much behavior hinges on this rabid attention-vying, but it's also impossible to deny the power when some otherwise-inaccessible influence is established. I assume this is why people continue to use it, despite the fact that 99% of tweets drop into the void.
[0] http://archive.is/Imikq