Why Did StumbleUpon Fail?
The service showed you a random new site relevant to your interests each time you clicked "Stumble". I used the service religiously back when it was up. It was my primary source to find cool stuff, which is a void that has only been partially filled by Reddit. Why did the company die?
Got passed back and forth to the founders in a series of acquisitions that failed to monetize traffic. An old story - (relatively) easy to gather a huge crowd, practically impossible to monetize it without destroying the product.
SU is now mix.com, I thought?