Social bookmarking sites can also imrpove your PR especially if it is using dofollow. You can have traffic on these sites but it does not mean that you can have also a better adsense income. Your income depends on the clicks from your visitors. Most of SB users today are very educated and they all know that you will receive dollars when they click your ads.
Most of the time, not ALL of the time, when someone says that about social bookmarking sites, it's either their site or their submissions to the bookmarking sites aren't all that great. First see if it is one or both by asking as many people as you can for their critiques, and then start all over again.
I had a page that was doing well for awhile... but I was still only getting around 1% ctr... the traffic is just hugely untargeted that's why you get horrible click throughs on ads.
I've had great success with adsense and social bookmarking believe it or not, I tend to get 3% click through from stumbleupon etc. Not sure how or why, but it works out well
You are just getting more targeted visitors from stumbleupon. My site was about mac w/ mac related adsense, but my visitors from stumbleupon were coming to see a video that had nothing to do with mac. If you spend time and actually get something nice set up, it could be a real nice source of income.
Most users of social bookmarking sites are ad blind ... however social bookmark backlinks are great for long term organic search results.