What's been interesting to see is the gradual migration back from Digg & Reddit; not so much HN. But the issue tends to happen when sites begin to try and cover "everything" that you end up driving out the original and central audience.
I actually stopped visitng /. for awhile there when I found digg would cover stories almost immediately, only showing up on slashdot a day or two later. But once digg got too popular their promotion system clearly broke down (not to mention the awful awful 'community', and I've been back to /. (and later HN) since
I actually like the slightly slower, curated speed of 10-15 posts a day, spaced out a bit, instead of a constant churn of thousands of things. Although, with HN you can get an algorithmic approximation of that via HN Daily. The main thing that's still missing is the "why you should care" blurb that's Slashdot's signature: not just a link, but a link with something about why it's being linked in the first place. I find titles to be a bit too cryptic for my tastes, and the use of only titles tends to promote linkbait titles.
Digg really shot themselves in the foot when they tried to make the site "personalized" - most in their audience wanted there to be just "one" primary home page.
Reddit seems to have been able to create many successful smaller communities around sub-reddits.
It's another lesson that trying to be all things to all people usually satisfies no one.
