First, I didn't know that eBay bought StumbleUpon last year. And then now, it is up for sale again according to TechCrunch. It is not yet known how much the selling price would be but eBay has already enlisted Deutsche Bank to find the right buyer. Below is a excerpt from TechCrunch's article.
In May 2007 eBay acquired website recommendation engine StumbleUpon for $75 million. Now, less than a year and a half later, they’re trying to unload it according to a source with knowledge of the sale process.
I used stumbleupon for a year and gave good results. However, it sends useless traffic to your site since most of the users are bloggers trying to get traffic-money. My bounce rate is at staggering 86%. But at least, it helped bring traffic to my site and a few friends who exchanged-link with me that in the long run, helped my PR ranking. Not that PR really matters but it is still some kind of status symbol. :)
But I was banned when I joined stumbleexchange in DigitalPoint. I never re-sign again after that since I can't really monetize the traffic that I get. Besides, almost all my contacts/friends in stumbleupon were banned, too! But if I find something new with them, I might try to join again but will no longer be active as before.