The top 20 social bookmarking sites are websites such as stumbleupon, reddit, delicious, digg, and even sites such as pinterest or twitter. If you are submitting to these type of social sites yourself, 90% of your focus should be on the top ones, to avoid wasting time. However, you can always use a service such as Bookmarking Beast http://www.bookmarkingbeast.com where you just enter your webpages and get all the top social bookmark sites as well as the hundreds of other tier 2 sites. Great for link diversity and increased search engine traffic.
