The StumbleUpon plug-in, supported in a variety of browsers, doesn't seem to be pleasing its current owner, eBay. After paying around $75 million for it about a year and a half ago, it seems they are suffering buyer's remorse and are looking to sell the company that develops it.
The primary reason seems to be “stagnation” in the number of hits provided by StumbleUpon. This is despite the fact that adoption of the plug-in has continued at a steady rate, currently installed on an estimated 6 million browsers. That's quite a lot of people to have use your plug-in, though clearly eBay doesn't feel it was worth the $75 million they paid. They may end up selling it at a loss, or even rescinding the sale offer if it doesn't go as they hoped.
StumbleUpon made a huge leap about two years ago, when they first offered multi-browser support.