digg is a user powered website where users vote on a story and if it gets lot of votes it lands in the front page (actually theres a lot more to the algorithm) If people dont like it they can bury it. a
LOT of people visit digg and if your (assuming you have a blog
) story makes it to digg front page most probably your server will crashe due to such a huge traffic coming to your site
while stumbleupon is like a lighter version of digg.. you need to download a toolbar and you can specify your interests.. if you like a website that you are currently seeing you can give a thumbs up and a thumbs down if you dont like it.. you can also write review for websites... and if you press the 'stumble' button you will be taken to a random site in your 'interests' that other people have stumbled (liked)
unlike digg, stumbleupon sends less but uniform traffic to your site for long term if you have good content that is
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