Submitting link to social bookmarking sites is good for driving traffic. Imagine the numbers of people visiting those popular bookmarking sites. They're in the range of millions. Just a fraction of these coming to your site is good enough.
However, for backlinks purposes, bookmarking may not be the best to go. Most of these bookmarking site have employed NO-FOLLOW policy. Hence, it wouldn't pass on link advantage to your site. For building of backlinks, it's useful to try posting comments at high ranking blogs.