Sometimes we may think we have great content, but we really don't, we as webmasters are often highly biased in favor of ourselves.
Also submitting your own stuff to social networks often goes against their TOS's, you can expect both your account and site to be banned from most of them if that's your plan.
Specifically Stumbelupon:
Incentive Programs
Digg's a little different I believe, I don't think they mind one or two diggs from your own site, but overdoing it gets you banned.
There's much better ways to promote and advertise than spamming social network sites.