I saw that alot of sites get hundreds of visitors each day from social sites. I submitted my article before week to 20 sites, digg reddit and more. I used alot of tags.. and nothing! I got only 2 visitors from digg! Why?!
Most times with social sites the only way to get traffic from them is to build up your profile within the community. A blank profile that hardly participates will not gain very much traction. One social site that works well for traffic is Stumbleupon but you have to stumble some sites first and post quite a few reviews of your own before you start seeing any traffic from it. The second best in my opinion is Digg because it indexes high on the SERPs for a day or two. The main reason for social bookmarking is for backlinks and building yourself up in the web. The more social sites with the same info to check your site against the more relevant your you will appear on the web.
Digg is worthless if you are not frontpaged. reddit is great if you have great content and submit to the right sub-reddit. but you need to be familiar with the community and their interests. You need to not only submit stuff from your own, but also other content.