High PR social bookmarking sites need to be on your to do list if you build links yourself. We use to submit to all the top tier higher pagerank sites such as Twitter, Pinterest, Stumbleupon, and Delicious manually but I was starting to do more SEO work than running my actual business. We turned to Bookmarking Beast http://www.bookmarkingbeast.com which gives us a report each month to the high pagerank social bookmarking websites they submit to. Not only do they do this manually, but they are very cheap which allows me to focus on other parts of my business.
