A lot of webmasters who once got to the top of reddit or digg, claim that it brought them a lot of traffic but no earnings. Like visitors come to a page but click no ads. As the result, you get low CTR, which is believed to cause low CPC.
The truth about digg and reddit visitors is that some of them even got ads disabled, and you can get about 2 or 3 extra clicks after getting to reddit top list even if your page is properly Adsense-optimized. So traffic from social bookmarking sites won't increase and can even decrease your earnings for a short perido of time.
On the other hand, getting several thousands of visitors per hour means getting a lot of backlins. So the main thing you can get from social bookmarking is not earning, but higher page rank.
With my other blogs i experienced that startegy and that was very helpful. I got about 500 backlinks for one page that got to reddit top 5 or something, and it was also submitted to digg, delicious, etc by visitors, not by me.
These days i'll make a couple of posts on how to get to the top of reddit and digg, so do not forget to bookmark this blog!