What is StumbleUpon?
StumbleUpon discovers web sites based on your interests, learns what you like and brings you more. Using a combination of human opinions and machine learning to immediately deliver relevant content, StumbleUpon presents only web sites that have been suggested by other like-minded Stumblers. Each time the Stumble button is clicked, the user is presented with a high quality web site based on the collective opinions of other like-minded web surfers.
StumbleUpon service shut down in June 2018 in favor of the new platform Mix : https://medium.com/@gc/su-is-moving-to-mix-c2c3bff037a5