Would you mind sharing your stumbleupon user name with me? I would love to follow you and your shares! :) I am beginning to get more active on SU and it is definitely paying off. I have had a few posts in the last several weeks that have brought in 600-700 visits each. :) In regards to your "challenge" I am going to try out paper.li I will let you know how it goes.
Beyond organic and direct traffic, you must understand the difference between all of your traffic sources and how traffic is classified. Most web analytics platforms, like Google Analytics, utilize an algorithm and flow chart based on the referring website or parameters set within the URL that determine the source of traffic. Here is a breakdown of all sources: