eBay has bought StumbleUpon, a site for sharing web sites, for around $75 million. The press release says it's grown 150% over the past year and "delivers approximately five million new recommendations a day to its large, highly engaged user base". What eBay doesn't explain is why. With previous purchases such as PayPal, Skype and Shopping.com, there did seem to be a rationale, but this one escapes me.