At first it didn't quite make sense to me why eBay would want to acquire StumbleUpon. eBay does auctions and sales. StumbleUpon does random entertainment. At least, that's the way I used the two sites.
But then, of course, I remembered that StumbleUpon is basically just a huge database full of associated interests. It's kinda like Amazon. People who liked X also liked Y.
Deploy that technology on eBay, and suddenly your users find all sorts of stuff for sale they never knew about. I imagine somewhere inside eBay there's a chart showing that a huge percentage of their site is basically untrafficked. StumbleUpon, if deployed correctly, would help.