Having 50 links from social bookmarking sites isn't go to put you at the top, but I personally think it adds a "credibility" factor, i.e., it gets your site's name out there more, which I think Google gives some type of weight to.
So if I was told that I could only have 100 backlinks, I would still use 10 of those links on nofollow social bookmarking sites. And as careerskk mentioned, it can help drive traffic, which is what your looking for in SERPs.
I dunno if I am getting a step beyond from giving specific answer to the question. But one thing I have learned is that social bookmarking links may not have a good link value / juice when relevance is concerned. For example if you have a link from an article, the entire page would have your keywords sprinkled here and there. This could add to link juice.
But as far as social bookmarks are concerned, links may have a very good value.