I think digg traffic is useless. however, if you're a popular stumbler with a lot of fans, SU traffic can be a good money booster. but make sure you have good content otherwise it'll just be a wasted effort(but isn't that the rule for making money!)
Social bookmarking sites gets you visitors but does convert poorly. But it's also a great way to earn some backlinks in the process.
I personally think that social networking traffic is in large part useless. The problem is that most of the time the visitors are tricked into visiting the site which really doesn't help in terms of conversions. I think search engine traffic is still the best.