I don't know why search engine gives so much importance to these social bookmarking sites over the actual websites. These websites always ranks high and they just provide link and snippet of original post. These social bookmarking links are changing the shape of search. I saw on Yahoo! buzz, which is a social bookmark sites, links to submit the information to other bookmark sites. So, if a post got place on Yahoo! buzz, from there, someone digg this bookmark and now when someone search on internet, Digg links comes, then from Digg, he will move to Yahoo! buzz and from Yahoo! buzz to original article. It's too bad
it's simply because DIgg is more respected than your own site and happens a lot. Think about it, they have hundreds of thousands of visitors a day, have content being added every few seconds and so Google give them more prominence as a popular site.
Digg is best to get some good exposer but it is not a daily traffic . If your post get popular than that can also help you in Google search engine too .