Hi Saul, I'm still learning how to get the most from StumbleUpon and it already runs in the top five referral sites to my blog. Not only is StumbleUpon a great source of site traffic, it's quality traffic with low bounce rates.
I avoided StumbleUpon for a very long time because it seems old and outdated at first glance. They could do a lot to improve to site, such as program the toolbar to call up more recent posts while stumbling, but that doesn't stop us from networking with some great bloggers there.
I have about five or six people that I send posts to, out of about 325 connections. Would you recommend I send to all connections, all of the time? Right now I just send to those I receive pages from.