Social bookmarking is a part of low link building, in simple words, it can be described as tagging a website and saving it for the later. Rather then saving them to your browser, you are automatically, saving them to the web. The best part of doing social bookmarks is that they are done are online, so, obviously you can easily share them with friends.
I personally suggest that while doing a social book mark, quality of the content matters. It should be relevant to the visitors need, who will going to visit therein future. Besides, try and post on high pr sites. First do it and see the difference for yourself.