Well Social Bookmarking is a great resource for traffic... but each website gives you different traffic types. To put it in simple terms, Digg traffic comes to read an article then leaves, they don't Browse around, same with Reddit, Fark, and Newsvine. Traffic from stumbleupon, the people will stay and search around the website, overall better traffic. Mixx traffic is the same, I like the community on Mixx and they have a high tendency of staying around and reading.
I recommend if you are going to be using Social websites, that you put Related Article links at the bottom of your post, I noticed that by doing this you will have a higher chance of people staying and reading other articles.
If you want to read a good article about the types of traffic to expect from these websites,
you can follow this link and read the article.
My favorite Social Submitting website is
Socialmarker.com, it's the best one I have found so far, and it's growing and changing on a daily basis. Just make sure you do not submit your own website to Newsvine, they are the only website that cares if you post your own content. Other than, use the websites that apply to your article, so do not submit an article about the environment to Sphinn which is all about Internet Marketing, or to Slash dot which technology news. So if you stick to those rules you are good to go and will get traffic from them.
Another piece of advice, the time you submit to these places makes a big difference, I recommend testing out posting times and see which one works the best for you.