In order for you to make good use of social bookmarking, you must submit your site to relevant bookmarking sites that are similar to your site's topic. For example, do not just submit your site to digg, because sites like digg only appeal to the mass market. People who want to get information on the same topic as your site probably go to a much more topic specific bookmarking site. Therefore, say you have a startup website that is a really cool idea and has potential to be the "next big thing", then submit your site to a site like www.mykillerstartup.com, because that's a bookmarking site where a lot of people looking for the "next big thing" get their news from. Or A if you have a new APP that you are trying to promote, then submit it to programmableweb.com, a site that programmers look for news.
Very good advice. Too many people get wrapped up in the raw numbers of sites like Digg, Stumble et al. without considering that the quality of the traffic those numbers represent is crap. If I'm promoting premium WoW addons, I'd rather my site be seen by 100 WoW players than by 10,000 general Internet surfers. Frank