Several things to consider before you go for social bookmarking. Firstly, make sure that the original content that you are doing bookmarking for is of good quality and not some cheap spun material. Secondly, ensure that you post on high PR sites that also provide do-follow links (to get good high PR do-follow sites Google the term or visit vmoptions). Thirdly, construct the title of your sbm with the keyword and make it interesting and not just some cheap publicity, and finally put the keyword in the description area too.
SBMs done in this manner can surely help increase rank.