Google Alerts seem to be a useful way of having a continuous search for certain topics. It is kind of a RSS feed to the Google search for that topic. The alert I set up was for Dungeons and dragons for which a new revision is due soon and I would like to be up to date on.
I Signed up on Digg and found a couple articles and voted/commented on some as well. I found that I like the idea of digg.com but it is not my thing. I would rather just look around online to find things and links from my usual sources than popular things which really do not interest me.
The article I posted on Digg was about physics and math which interests me but I doubt such a dry source would gain much approval on digg.
I can see why these sites get so much net traffic though. with so much content on the web may users could get lost in the sea of information and using these sites to organize sites and vote on thinks they like can allow for easier web browsing.