How Stumbleupon stumbled...

In a nutshell, it's not social.

And it wasn't Facebook and Twitter that did it in. It's Reddit and Tumblr:

StumbleUpon recognizes that it needs to keep tweaking the service in order to retain current users and recruit new ones. Post-redesign, the product seems like it could use a little love. While browsing from the web, we hit two defunct pages; meaning that instead of serving up a funny or informative website, StumbleUpon took us to the placeholder page of text ads. The private message feature doesn't work. The site relies heavily on a survey users fill out when they register, coupled with behavior while using the service. In other words, it takes a lot of input to train StumbleUpon to give you good recommendations.

The experience is also somewhat isolating compared to other social media. "I didn't really need yet another discovery method, on top of Reddit and Tumblr," Nick Douglas, the editor of humor site Slacktory, said when asked if he uses StumbleUpon. "Both of those include decent discussion; they have an actual culture. [StumbleUpon] seemed like a toy I was using, alone."

BuzzFeed is a bigger fan of StumbleUpon.

I expect BuzzFeed will end up being the next acquirer of SU.

Tumblr is to StumbleUpon as Reddit is to Digg.

Tumblr and Reddit are about self-expression through posts and comments, respectively.

StumbleUpon and Digg are just buttons to press. That gets old after a while.

Self expression never gets old. Hence the popularity of YouTube, Blogs, and Twitter.

Totally agree. Much easier to find a place with better content than a place with a better community.

Content is easy to find.

A community that makes you feel content is much much harder.