Prior to the Internet you had mass media controlling distribution. The Internet comes along and you have things like Usenet and the proto-Web.
Then comes the first crowdsourcing sites to allow people to find content without employing people to curate that information. Slashdot was certainly early in this trend.
What Digg allowed is a certain band of people to control the information flow. People would get paid to promote submissions as it became clear that a front-page submission generated a lot of pageviews.
But what became apparent with all these community sites (and this includes forums) is they start with an early group who provide value to each other. This group ends up becoming insular. De facto standards form. But even in the Usenet days you had the "September" problem (where new college freshmen would get Internet access and not understand the "rules" and conventions that were in place and would ask questions that had already been answered, etc).
Basically all these social sites get worse over time as the masses flood in.
Digg died because the idea that there is a central source for news was a holdover idea from the old media days. Reddit understood this. Global reddit is basically useless. The subreddits are the only remotely interesting thing about reddit.
People complain about how HN is getting worse. That's probably true and it is true (and will continue to be true) of any such social site in the future.
I've heard the same complaints about Twitter.
Facebook for most people is not a source of news. It doesn't have the same link-sharing mindshare (IMHO) for most people that other mediums have. Ultimately I think the biggest use case for Facebook is still sharing photos. People go to Facebook to find out what their friends are doing. Very few go to find out what's going on in the world (much as Facebook would like that to be the case).
I'm surprised at how some wax lyrical on how amazing reddit is. It's just a minor tweak on a long trend of existing prior art (the subreddits). Personally I think it's a cesspool full of trolls. Proggit (programming.reddit.com) is (IMHO) just awful.