I'm a HUGE fan of social bookmarking and have done many tests with using only SB on brand new domains to see very big gains in SE rankings for my targeted keywords.
People who say it doesn't work either:
(a) only used nofollow sites
(b) didn't wait long enough for Google (or Yahoo, etc.) to index the pages where their bookmarks appeared, or didn't ping those pages
Plus the new Google Caffeine search algorithm seems to favor social bookmarking even more than the current algorithm. Every one of my niche sites I've checked, which has a lot of social bookmarking backlinks, has jumped in rankings. See for yourself:
Compare Google Caffeine
I use Blog and Ping Tool - Use Pingler.com to Drive Traffic your Blogs and Websites (similar to Pingomatic except you can add your own list of services).
As I said above, absolutely yes.
Without getting too technical...a backlink is a backlink is a backlink. Social Bookmarking is powerful because a single bookmark creates multiple backlinks (one on your user page, one for each tag, and if your page is bookmarked by many users a lot of services have a "top bookmarks" page with high PR that you could receive a backlink from).
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I swear, by my life and my love of it, that I will never live for the sake of another man, nor ask another man to live for mine.
I assumed since the target audience of my post was questioning the effectiveness of social bookmarking in its entirety, they had no knowledge of the PR formula, NoFollows, or robots.txt.
Which is why I said "without getting too technical". Because all other things considered equal, a backlink from social bookmarking is as valid and effective as a backlink from article marketing or a forum signature or a blog post, etc. etc.