You know, I'm really unconvinced that StumbleXchange is a good idea. The whole point of StumbleUpon is that it's built on recommendations from real human beings - people stumble good stuff, stuff that they like, and an algorithm sends the same page to other people who it thinks might like it. When you introduce a lot of people stumbling random pages simply in order to get stumbles themselves, you're bound to have an adverse effect on the system as a whole, and on your own reputation within that system.
Check the reviews of StumbleXchange:
StumbleUpon » stumbleXchange :: Exchange Stumbles to P...
As one reviewer summed up in just three words: "Feeding on itself."