This was a hard Thing for me, because it doesn't fit very well either into my personal use, or into my current work use.
In my personal life, I tend to get most of my news from radio (and, perhaps predictably, from Minnesota Public Radio), and from reading the newspaper on work days. I read very little online news content unless someone else links an interesting story, or unless I'm looking for something specific.
And at work, while we certainly highlight occasional news stories of interest, we don't have a single point of contact for students (other than the school website, which logically focuses only on stories or news directly relating to students or the school, which tends to mean school sport related stories most of the time.)
I did play around with various of the sites (Digg and Reddit, more than the others) but I found them frustrating because it's hard for me to filter for information I want in a time-efficient way. For example, while I'm interested in health issues, do I really need to browse 15 stories to find the 1 or 2 that have new information or content for me on a topic I'm interested in?
It's far easier for me to find that through searches, or through subscribing to an RSS feed for a blog that generally highlights news of specific interest to me. Thus, using them seems to be a productivity detractor: I could probably set things up in a way that would be effective for me, but it would take significant time to focus what I was seeing - and I've got easier ways to get there.
In a library setting, I can think of some options for use, though (as I said), not so much in my current job. Certainly, some people who enjoy general news browsing might find the tools useful - and people who want a range of news sources would also benefit.
The question of popularity:
One thing I wonder about with these sites is the popularity/vote driven aspect. The same information is available on del.icio.us and tagging sites - but in that case, the popularity alone isn't the only entry point, as you can also find connections through a particular person's chosen links, or through related tags.
I'll be the first to admit I am not mainstream about my interests or preferences in some ways (especially when it comes to information sources). I'm much more interested, for example, in a fairly detailed commentary on medical issues from a hospital (not only in terms of content, but in terms of writing style, focus, level of detail) than an article in a popular magazine (I found several Reader's Digest articles in the searches I did.) However, the latter ones are far more likely to be popular with more people, and so will tend to rise to the top of the list.
Part of being a librarian, for me, is, of course, knowing that my preferences aren't everyone else's preferences. (I've known this for years: the amount I read for pleasure - and *what* I read for pleasure, which is heavy in the SF, Fantasy, and Mystery genres, was sort of a dead giveaway.) At the same time, adding a method that's inefficient for me (as I suspect any of these sites would be, both short term and long term) is perhaps not my best choice.