you have to ensure that these sites are dofollow links otherwise it' a wasting time
Personally I depend upon SE for traffic as far as social sites concerns DIGG and TWITTER really help me in getting traffic.
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Well I use many Social Bookmarking Sites for getting traffic ..
and many more
but Digg is my favorite Site..
Well, I'm sure Digg brings the most traffic on you website if you get to the first page but it is quite hard to do. I usually don't get any traffic from bookmarking websites because I submit my pages for backlinks.
Follow me on Twitter: @hendricius
All the social bookmarking sites be helpful to get more backlinks to the site. But to attract more people to you, you need to provide unique and useful description in your submission so that you can get attention of more people through your description. And the other need is to be more focused in choice of the titles and meta tags through which your content and post is going to find by people. So titles and description are the most crucial things in your submission to get better results than other.
this time i get most traffic from StumbleUpon only. coz i don't use other social media site. but earlier i used to get traffic from yahoo and digg.