To start with Bookmarking is of no use in terms of link building, at least not directly useful. However they can surely send in some traffic, provided the content is useful and attractive. However one must remember that Book marking will never work for Sales pages, in other words product or service pages. Book marking works perfectly for blogs and articles, (again, provided the content is worthy).
, Reddit (most important), Digg, StumbleUpon and Delicious are among the best bookmarking sites and if used effectively they could bring in a lot of traffic.