The best social bookmarking tool to use... for me.. is socialmarker.com. You can submit up to the top 160 social bookmarking sites at the same time FOR FREE! Also twitter is an awesome tool for internet advertisement as well. Social bookmarking sites are a great option for obtaining organic traffic . Organic traffic is traffic that doesn't come from search engines and doesn't cost cash.
Also generally speaking, social bookmarking is a more trusted from of advertisement because the "ads" are being presented by "genuine poeple" and on twitter they are even you "friends" that you are "following". So of course many poeple would be inclined to click on something that other people are recommending before they click an ad that is popping up on their screen or on the side of a web page.