StumbleUpon sends a lot of traffic to BlogStorm and it is probably the easiest of the social networks to use.
Daily Moolah has a guide on Leveraging StumbleUpon that tells you everything you need to know about Stumbling.
The one point I don’t agree with is trying to manipulate StumbleUpon in public. Posting your url on sites like Digital Point is asking for trouble. Hundreds of users get banned from Digg every year for asking for Diggs on Digital Point and it wouldn’t surprise me if StumbleUpon start taking spam more seriously.
If you really have content worthy of a Stumble it doesn’t need much help to get a few thousand visitors.