StumbleUpon was created in 2001 by Garrett Camp, Geoff Smith, Justin LaFrance, and Eric Boyd in Calgary, Alberta.  StumbleUpon has a web site, but it is primarily known as a browser plug-in and a toolbar that allows users to rate the sites they visit.  Particularly interesting sites naturally receive more votes.  Others users click a “stumble” button to see a random web site.  As users vote for particular sites, StumbleUpon becomes better at matching your interests with new sites that are submitted.  Therefore, the service becomes better the more you stumble, as it is more in tune with what you like—based on your past votes and your friends’ interests.

StumbleUpon is purely user-driven, since all of the sites are submitted by users and the popularity of sites is wholly dependent on how many times it gets a thumbs up or thumbs down vote.  StumbleUpon is unlike most other social sites, and its popularity (the site has over two million registered users) has made it a great success.