Alright, so basically a link with rel="external", has nothing to do with it being nofollow or dofollow, right?
Here's a sitepoint article on the subject, New-Window Links in a Standards-Compliant World, though just google it, and you'll find many more, most just saying the same thing.
Of course it depends on your articles and niche,but social bookmarking can bring in a lot of traffic and if the sites are do follow even help you with SEO
However many people are resorting to methods to get their own sites manually and intentionally bookmark to raise their ranking.
If you get on the first page of most major social bookmarking it has the potential to actually overload your servers that's how much potential social bookmark has.
It's like free advertising when I think of it if you do it properly.
good information = traffic. social bookmarking is a very effective way to boost your traffic but without posting good and informative news, people won't click on your link
i done a new site with out back link building, only i done is 100 social bookmarking (most of it with no follow tag).
it does bring me some traffic from SE, after a week 10 daily uv i receive from google SE now .....
so i think social bookmark does bring something, especially on popular niche ...
I've never socially bookmarked any of my web pages (apart from via Twitter sinceI am informing my followers about something new) since I feel that is what other people should do to recommend the page.
But if you produce content to be proud of that you are sure that others will be keen to see, then I think it is a good idea to socially bookmark that content under an alias (pen name).
But I have a hard time justifying the use of a plugin that automatically socially bookmarks every post that you may publish to numerous sites, or even worse, every post that is automatically added to a site where the content has not been seen by the webmaster.