stumbleupon.com is the most important one. Because it is a search engine and people love to search here because it return exact and narrow result not like google which return worthless pages also. If you will type straw hat in google the top 3-4 results are related to straw pizza shop which is really irritating.
Social bookmarking helps a lot and gives you quality backlinks. But you need to search for good PR bookmarking site like slashdot.com it really helps. From my own experience.
Social bookmarking definitely helps to generate traffic to your blog specially stumbleupon and hacker news. Stumbleupon’s url shortening service generates even more traffic.
Just doing simple bookmarking means you are just putting your site link there. It does not bring any traffic. Use sites like Stumble Upon and Digg and make your own community there so that when you do a digg , there should be people who dugg that post or re tweeted that.
As a matter of fact, you are not wasting your time. you will actually have to find the best social bookmarking site for you. Reddit.com and slashdot.org are worth discussing
Social bookmarking submission is the best way to get traffic and If you are not getting traffic from this that means you are not using traffic generator keywords.I suggest you to research on keywords and use some traffic generator keywords.
Always try to do work on Do Follow websites. Social book marking in do follow website is best SEO technique to build good back links, improve page ranking in search engine and boost traffic your website
Social bookmarking doesn’t generate results overnight. You may need to wait for a certain period of time before seeing good results. Also, it is advisable to implement other website generation techniques to increase your site’s visibility. Relying on one technique alone won’t get you the desired outcome.
I think social bookmarking is a great addition to any blog. I’m not sure how you can generate revenue with it, but I think it is a necessary tool embedded into your browser.