do both as submitting newer content to same bookmarking sites help keep the interest on your site of people. and submitting it to newer sites helps in SEO as well as newer audience.
You can submit your new content to repeated social bookmarking sites. If the social bookmarking sites have good value you can continue submitting them to the repeated social bookmarking sites.
I agree with the above comments, you can share same content with social bookmarking sites and will be good for seo if you submit to more social bookmarking sites with different content as well.
instead of submitting one link on each website, spend more time on social bookmarking websites, when some one got followers and have friends then the submitted may even get some clicks from social bookmarking websites.
Well social book marking is used for mainly getting traffic so if you are getting your targeted traffic from these sites so continue with it other wise you must choose other sites also for book marking.
I ditto akhomiyar. Continue building your readership at the current social site and add new ones when possible. On the other hand, don’t forget your own site. Like webcosmo said, try to increase your incoming links.
start bookmarking them, Its called indirect promotion method, It will help them to cached and traffic will be redirected from your article to your website.
Only add links that people would actually want to share, otherwise you’re spamming these sites and you’ll gain absolutely nothing from it apart from a bad reputation.