is it useful for page rank increase and in wich way to post a directory on bookmarking sites like delicious?
It will provide a backlink from a quality authoritive site. I doubt it would effect pagerank, but you should stop worrying about that.
delicious is no-follow so page rank etc will not be affected! social bookmarking is mainly done for gaining quick traffic! if you submit nice articles or press release etc then you can expect some natural backlinks due to some link-bait etc, but if you submit directories to social bookmarking sites, i don't really find any other benifit other than getting traffic.
Tt can't hurt...the more links and potential for exposure the better. It only take around 20 seonds of your time, right?