Thousands of people have put social bookmarking to powerful use for off-page optimization plus various marketing strategies. There appears to be no let-up with their reputation, especially for off-page SEO purposes. But you really should not neglect or overlook what could be done for the more long-term approaches at high PR bookmarking sites. Social bookmarking sites basically give you a place to keep bookmarks, for those who are really new to IM. You can assign tags (keywords) to your bookmarks, and that basically has SEO benefits but a lot depends on the particular site.
Google took a great deal of the power out of high PR bookmarking sites when it appeared they (perhaps) let the various sites know that their dofollow link attributes were causing issues. Places like Technorati and Digg, plus a few others, were able to rule the first pages of SERPS in Google due to their high PR status. Naturally, I make conjecture about any impact Google had with those changing their links to nofollow. However it is interesting, and we all realize how Google is or can be. You’ll find a few more strategically minded marketers who make an effort and create high quality profiles. Profiles that have appearances of being legitimate will do a better job for you and have a good chance of passing a review. There is SEO value with a great RSS feed; not to mention RSS directory submissions you are able to do. Consider the added benefit of more coverage for your links in those feeds. Just be sure you provide excellent content in your feeds and stay clear of being spammy with the links in them.
There’s a balance that you have to hold with the amount of content and various promotional methods in your feeds. Another important aspect is to take time and build relationships at these bookmark sites.
There is flexibility inherent with dealing with bookmarking sites; so how well you make use of them is your business. Most people are likely to just use them for backlinking, and it does appear that the approach outlined above is not applied by so many people. However, there really is strength in that strategy based on the feedback we hear from those who use it.