I find most of my traffic is actually from Social Bookmarking Websites.
Daily Breakdown
80% Social Bookmarking
10% Search Engine
10% Signatures
Same with me. I think it's because I am and my search engine ranking isn't all that good but hopefully as I get bigger my search engines will bring in more traffic.
Social bookmarking traffic does NOT click ads and hardly even stays on the site for more than a few seconds according to my counter(not really accurate) and so is about useless traffic if you want money. If numbers on a counter is what you want then you can get that.
i agree with cowsgonemad. i got a big traffic influx one day somehow from stumbleupon. my impressions that day went up 400 more than usual....and only about 20 percent of them stayed on the site for longer than 1 minute....