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Yes it is helpful. Not only for your blog, but for all kind of websites. Nowadays we can't live whithout social bookmarking and social marketing
it is helpful traffic wise but it can only generate a little quality traffic unlike what search engine gives backlink wise it also give some quality backlink depends on the site of your socialbookmarking sites.
StumbleUpon is quite good for getting a flurry of visitors if someone stumbles one of your articles. If you're using StumbleUpon, it's an idea to build your friends list to a reasonable size, then stumble your best posts. Not all at once, but intersperse them between other articles you've discovered. For example, discover 3 new articles, give the 'thumbs up' to a dozen or so articles and then submit your own article. Not an exact science, but hey-ho! Good Luck, John.
Some of today's reputed bloggers used this technique to get to where they are now. You gotta understand though what article to submit to which ones.
social bookmarking is very successful I find digg and redit especially good, although the quality of the traffic can be poor