The reason StumbleUpon is so hot right now is because it represents a fruitful area beyond search: discovery. Search engines are good for when you pretty much know what you are looking for. But discovery engines like StumbleUpon's use a combination of your own stated preferences and those of people like you to surface Websites, videos, photographs, and other information you didn't even know you might like.
That's why eBay would want it, and why Google might be miffed for missing out. Think of StumbleUpon as a Digg for Websites. As StumbleUpon's database of the best content on the Web gets bigger and bigger, it can start to carve out discovery engines for specific sites.
The fact of the matter is that eBay is facing some major disintermediation from Google (GOOG) right now. Many of its biggest sellers are setting up their own Websites and buying keyword search terms on Google to drive traffic directly to their online stores, bypassing the need to pay eBay a commission on each sale.
If discovery is the next of search, buying StumbleUpon could be an attempt to get in front of that curve, and maybe some day disrupt Google's business down the line.
Discovery can also be thought of as the latest twist on recommendation engines. And every e-commerce site needs one of those.