It depends on the search engine if they still index your backlinks on social bookmarking site every time they update backlinks.
According to information being released from SES that I have been reading just about everywhere, social bookmarking is a very good method to be using in terms of SEO. No doubt, seeing this method being abused, the SEs will find a way to tweak their crawlers over time making it less valuable. Look at MySpace for example. The Can-Spam Act has now reached to spamming of the bulletins and such on MySpace. See http://www.google.ca/search?hl=en&q=myspace+Can-spam&meta= for more details on that. So you want to make sure when using social bookmarking you do not over due it.
Most bookmarking sites use the "nofollow" tag so they can maybe increase your traffic directly from their site but it does not count towards improving your backlinks for SERPs.
most of the social bookmark sites are using nofollow in their links. you must check nofollow tag befre you decide that link will help you or not
I think social bookmarking is important for Traffic more than SEO ! It can bring you huge traffic fast and PR slowly !
I do agree but sometimes traffic becomes more important than PR so have to rely on social bookmarking at times
Depends if they set it in "nofollow" or they filter the site from indexing the links using robots.txt
treat social bookmarking backlinks the same way you treat press releases. it's all for exposure and a quick traffic boost. but it never hurts to vary your anchor text for various keyphrases you're after (short and longtail)
please don't use any of these so called "social bookmarking services" all they do is spam digg, delicious, reddit, ect.....they get your links removed fast for spamming