None. Links on social bookmarking sites have been devalued by Google, so they are not much use as backlinks. If you can find a social bookmarking sites where real people actually participate and will be interested in your links - i.e. one that hasn't been overrun with webmasters promoting their work and nothing else - then you may get some real readers from it, and that's worthwhile.
Otherwise, all the advice you see about promoting on social bookmarking sites is out-dated