most social bookmarking sites add a nofollow tag. They are great for traffic, but most wont help SEO. But there are exceptions.
try to install google analytics to know what keyword which bring more traffic to your site, then optimize your site using that keyword
All are about links! But what kind of link google wants to get on # 1 position. I do trade links in related sites with good PR but still I can't get in # 1 for certain keyword. I studied the back links of the site all came from 2 news sites but not related. What does really gives a big factor to get on top?