This is a little technique you can use to get that extra credit needed to get to the top of google for those harder fought keywords.
What I mean by this is that don't just spend all of your time with 5 to 10 social bookmarking sites, marking everything you create. Take a little extra time and create a couple accounts that don't necessarily point to your site but to some random sites that have similar interests, then point them to a couple of your bookmarking pages.
Doing this periodically should help boost you up to the top.