Twitter! It is so fast & great for my niche (bargain blogging) when deals pop up faster than I can post.
I wish I could figure out how to use Stumble effectively. I have no idea how to check to see if I have ever even been stumbled, but have a link on my site so if you'd like, feel free to stumble me & explain how I can view it. lol.
For me, it is RedEdit when I remember to use it. The one I have the most difficultly with is Propeller. I can't log into my account - I have two now and neither is working. I use the others listed above but not Plurk - I hadn't heard of it until recently.,
I'd have to say Twitter.. And only Twitter for me.. Other than that, I hardly see any hits from other booklmarking/networking sites:-(
By the way, if you wanna follow me on twitter am http://twitter.com/themomwrites. I'll follow you back:-) And if you want to check out my blog. It is The Mom Writes.
Am really happy, though, to have found some great advice and tips here!
I would have to say plain old Google searches still get the most, with Twitter being next. When I write something really, really techy that I know others will probably want to know, I stumble it myself and that usually brings on a ton of hits.
One thing I noticed that is strange though is that my traffic counter never shows the right # of posts that come from SU, so I'm not really sure about that.