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A blogger has done extensive research into the StumbleUpon algorithm to see how exactly the service works. According to the researchers, each Stumbler has an audience score that consists of their number of fans, the numbers of thumbs up/down they have given, and bonus points if a user give thumbs up to content that has already been given approval by other users.
From there, the researchers hypothesize on a number of possible algorithms based on audience score that StumbleUpon may use. For example, if you are the first person to stumble something that ultimately gets a lot of thumbs up, it increases your audience score and thus how much weight you carry in the system (similar to the Digg algorithm). The researchers also speculate that arriving on a page “organically” (i.e. – not via a stumble) also carries much more weight.