StumbleUpon is great site to get traffic but the traffic comes from stumble upon is not much quality. This are the just stumble upon user who are stumbling to surf and find new and interesting site. You can also use paid option from stumble upon to get more traffic.
Stumbleupon is one of the most popular social network. All Social networks, including stumbleupon are very usefull for getting traffic to your site for free. I have website for intresting and funny image, I'm getting avg.368 visits to that site only from stumble upon.