I haven't really used Stumble (except for su.pr occassionally) for a while now, but I think there is some value in using it occasionally, especially if you have a big post: like a new ebook.
If you want to exchange stumbles, my email is christine.gilbert@gmail.com. Just send me an email with the subject "stumble" or something like that and the link in the body of the email. I won't email you like crazy-- no more than once a week, and more likely less than that.. Feel free to send me whatever.
I use email, because I don't really like stumble's share options.