StumbleUpon is a search engine that recommends and directs users to webpages. Users create an account and choose "interests." Users are guided to webpages that have content relevant to their "interests." StumbleUpon allows users to find photos, videos, and other online content that they would not normally see.
Unlike many social media platforms, the primary objective of StumbleUpon is not to interact with other users. Instead, users interact with the site: choosing interests, giving feedback on suggested webpages, and occasionally viewing trending webpages. The only interaction between users occurs through Facebook. Users can invite friends from Facebook to join StumbleUpon and follow other users in addition to their "interests."
The main function of the site is to generate websites based on each specific user's interests. By clicking on "Stumble", the site brings up a new page. The user then has the option to click the thumbs up or the thumbs down buttons, which the site then incorporates to determine better what the user is interested in.
StumbeUpon was purchased by Ebay in April 2007 for $75M.  The purchase essentially demonstrated the popularity of the search engine and the potential attraction it held. In a sense, it also demonstrated the large number of "mindless" web users who would rather be directed to pages they like than search and explore themselves.
Community and Identity
Although not explicitly recognized, there is a Stumbling community. Those that Stumble are a part of said community and those that do not, are not. Online, the Stumble community consists of over 25 million users who collectively stumble over one billion times per month. 
User identity on StumbleUpon plays a key role in the user's overall experience with the website. A user crafts their identity by the "interests" they choose. Because there are thousands of interests to choose from and an unlimited number of interests each user is allowed to have, almost no two users have the same interests, creating a vast and diverse community of Stumblers.