The bookmark sites will give you backlinks towards your website which will help even though they are not related to your site. It is a free one way link and if the members like the content of your stie then it can be voted to the front page of these sites a link on a
pr 7-8 site usually go for about $100 a month so it would be worth it to place your link on the site. The people that use these sites are not looking for a certain type of site so they may be interested in your niche.