Several sites are working on AJAX scheme, so the links are not visible to search engines, so no SEO value. However others are SEO friendly. And about traffic I think it's targetted traffic, but mainly are tech people who usually don't click on ads.
It's very individual and depends on the niche and your tactics. Social bookmarking can do good if you do it carefully and thoughtfully. Here is a Seomoz article which can help.
I found an interesting idea today... By tracking your social bookmarking compaign you can get some idea what really interests your reader. Source "Drilling down and finding all the referring traffic data from one site is a really cool for a variety of reasons. For one thing, you can track your traffic from a social media site in detail. Here is an example using StumbleUpon: As you can see, we have 3 peaks in the data, and these peaks relate to the days I published 3 specific items of content, the first being an interview with Adam Lasnik, the second being the web analytics report already referenced above, and the third being a post about 17 Poor Quality Signals Your Site May Be Sending. The cool thing about this is that you can quickly get a strong feeling for what content a particular audience likes best. Mapping your content performance across social media / social news sites is a good way to learn how to better map your social media campaigns."
If can drive additional traffic to your site and also additional backlinks. How ever submitting to your site to social bookmarking site is good if you have quality site.
I'm not very sure of the SEO value of bookmark sites, but i am very sure of their potential traffic value . A good and unique article can provide massive traffic, hence some may link you back , other then that it provides next to nothing SEO value.
I feel it is based on the uniqueness of the article and site. Just like sharkyx said, if it is good then you may get backlinked if not it just may sit there and provide little SEO value...but it doesnt hurt if you have the time.
I think so because some of social bookmarking site are counted as backlinks by search engine. It can also be a source of traffic.
SEOwise? I don't think so. It can bring you traffic for sure but not the kind that can bring you profit...
I think on SEO part social bookmarking plays a minor role.I used DIgg and stubmleupon and it helps to bring my articles on top 10 but not always.you cannot bookmark every articles so it dont have a big part in SEO.
it will help your seo but yu must check nofollow tag before you do this and there is one more which you must allways remember. if you spamm those sites, you may be banned