No matter what the tool (social bookmarking, social networking, SEO, etc.) the best way to get more traffic is to focus on getting one target visitor / reader and focusing on them.

The more you think in terms of just "traffic" like herds lining up to come to your website, click your clinks, buy your product and do some general grazing, often the less long-term and lasting success you will have.

I say, focus on getting one good reader. Do this by understanding just exactly who that reader is--what they want, what they need and how they'll interact with your contact. Provide a good experience for them with quality content that makes action easy (whether it be commenting or low-clickthrough sales) and welcome their interaction. When you get the interaction of one person and you please them, suddenly all the efforts you were doing with social networking become their efforts. You may tweet till doomsday about your new blog post but if someone else tweets about it, saying they've commented or they're reading it or just that they liked it will ultimately attract more attention and interest more visitors / readers / traffic.

When you've converted one good reader like that, you perk them up to whatever follows, too, so it is good to have your best foot forward on your website. Have plenty of accessible, free content and make it consistent. Your reader will return, will bookmark, will subscribe to the feed of a website that did what they wanted it to. So you just have to figure out what your reader wants, just that one reader, and provide it. They'll help to bring traffic in for you.

That's my $0.02. I don't think much of anything beats word-of-mouth and referrals. SEO may fade, and what's dugg may get buried. Creating lasting impressions of satisfaction with your visitors is key to getting more, qualified traffic.