I would have to say that Stumbleupon brings me hundreds of visitors daily with Twitter coming in second and Kirtsy coming in third.
I've never experimented with Facebook, don't like Plurk, and don't even know what Mebo is:/
I'm pretty green (under a year) to blogging so I"m hitting them all up, but not heard of StumbleUpon...so I'm off there right now...thanks.
Stumble upon brings me the most traffic when I submit my content it too it. Followed by twitter, I use facebook but no one seems to venture to my blog from there.
I clearly have no idea how to use stumbleupon! I have the toolbar installed and I stumble and discover and review, but I get NO traffic from there. No one submits my content unless I submit it myself. I don't think my readership is that into stumbleupon though.
I get most of my traffic from facebook though.
Facebook is where I started and has always been good to me. Twitter was slow at first but it seems to be coming around. Linkedin has also been very positive as well.