(Newbie here, living fearlessly and jumping into an involved discussion while still wet behind the ears. No guts, no glory, I say.)
I see validity in Receptional's comment earlier that "the nature of most blogging systems is that the linkjuice is short lived," and I've seen that with a blog I had in a previous life. Amazing burst of traffic when you strike a chord followed by rapid decay, especially if you're dealing with stuff that has a short shelf-life.
But I'm still wrapping my brain around the implications of the social sites. MySpace strikes me as being a remarkably closed system. I've seen external links from MySpace pages, but they appear to be the exception more than the norm, and MySpace users appear to operate for the most part within the confines of MySpace.
As social bookmarking sites go, I owe my recent explosion of traffic to being picked up by StumbleUpon users. I didn't do anything to attract StumbleUpon's attention...someone just liked my site and the ball started rolling naturally. This particular service is interesting because it puts your page in the faces of people who want to see something they haven't seen before.
Having seen the rapidity with which blog-based traffic can decline after the fires have cooled, I have been watching my site's traffic with more than academic interest since it was "Stumbled Upon." But going three weeks into the deluge, it seems to be holding steady at present.
My sense is that the category "Blogs and Social Sites" is a pretty broad one, that there is quite a bit of variety among the different species of these animals, and that the devil is probably in the details of how each player in this particular space works.