Here's a very interesting article that I think people should read. It's on one of the StumbleUpon groups. Stumbleupon's integrity = Gone? Here's part of it:
Stumbleupon is huge though- of course it's going to have a cetain component of spam. The only way it can be controled is if people using the service actually mark it as spam. I"m not sure what people are expecting?
There are many people that use StumbleUpon to market pages on their sites and get a bunch of traffic. At the same time, there is a very faithful following on StumbleUpon that hate spammers. The thing is that it's those of us that abuse it that are bringing it down for everyone. Ofcourse there will be some abuse, but it's reached a point where the avid Stumblers are very annoyed. The best thing you could do is have a good fan base and not abuse the system. Every so often a friend could review a quality page of yours and hopefully you'll get some relevant traffic.
Good link! If I could undo some of my post in the stumble threads I would not be in their sandbox. I would think by now that they stumble exchanges would of died off, once they catch you your traffic dies to nothing.