As the moderator of a social bookmarking site (soLinkable), I constantly see people using it incorrectly. They just spam their links, then complain that social bookmarking doesn't work. Well, I say they're doing it wrong. If you spam your link to 150 different social bookmarking sites, what incentive are you giving anyone to actually visit your site? Or to care about what you have to say? The only way you're going to see real quality traffic is by actually getting involved in the communities. This doesn't mean you need to actively contribute to 150 different sites. Pick out a few of the smaller ones and actually JOIN the sites. Vote. Comment. Contribute. That's the key. You will start to see better backlinks AND better quality traffic. I suggest you check out this article: How to Set Up a Domino Effect of Traffic. It's a great intro into understanding how to get social bookmarking to work for you. Good luck!