The proper use of social bookmarking sites isn't to increase traffic. It's to bookmark sites you find interesting and share them with other web users.
Bookmark quality content, and others may vote for your bookmark and give it more exposure. On the otherhand, social bookmarking can improve your offpage SEO and help you rank better. Just be sure to use your targeted keywords in the title of your bookmarks. But don't make it sound spammy, make it appealing to both people and search engines. By the way, most social bookmarking traffic is no/low converting. Even though you may get thousands of visitors, generally speaking they don't buy or click much.
Even if you get traffic from these sites it is observed that traffic is not converting to sales.. so thats kind of useless traffic for some..
Social bookmarking is best used for off page SEO and not for direct marketing because most of the people are just surfers and not buyers, so be prepared for a higher bounce rate.
Really, you should add the bookmark buttons to your actual content page so viewers actually use them, this promotes more traffic and allows you to get return traffic as well. From an SEO stand point social bookmarking is a dying method; due to most of the services now using the nofollow attribute.