When I have a new post reviewed on StumbleUpon, I can receive up to 2,000 hits in one day. But SU is fickle and will sometimes just not hit for you.
Hands down, the most reliable social network for building true readers (not random hits) is Twitter. For me, that is.
Friends are referring friends, you can develop a sense of the blogger's personality, you see how they interact with others (or don't= very telling), and you can keep up with their new posts without constantly checking for updates directly.
Most specifically, I give a lot of weight to who my friends follow on twitter and will click through to new blogs based on reading replies between bloggers. I found MBC by clicking through a new follower's link on twitter, went to her blog, and found this badge.
Like Catnip, I followed Velveteen Mind here too... so Twitter does "work." (I also got my very first readers from an awesome blogger (Ninja Poodles!) tweeting about my blog after she visited and commented. No one has "StumbledUpon" my blog just yet, so I can't say anything about the other networks.
I would have to say Twitter has worked to get people to me. Also, MBC has been great. Communities like this are what drive ppl to my blog and give me loyal readers. Honestly, Facebook I am not part of and note really interested in. I don't like Myspace, and StumbleUpon has a mind of it's own sometimes.
I followed Velveteen Mind here, too. I don't get a whole lot of hits right now because I'm still building my mom blog readership. I started in political blogging so more people know of me through that. One of my posts was reviewed at Stumble Upon and got a lot of hits from that, but haven't had any luck since then. I'm not sure how to use it.
I would like to add Kirtsy to my posts, but I haven't figured how to do that.
I am so behind when it comes to social networking sites. I signed up for twitter but didn't know what to do with it. I need to spend some time reading about it and how to make it work for me. I get some traffic from here and some from the parenting board that I belong to. I have to say though, that there isn't any one place that I can see most of it coming from. Wish I did, then I could target my promos that way!