In Adsense terms, the traffic from digg and reddit, for example, is of low quality. The visitors from these sites are invariably just 'surfing' and are only visiting because one page in your site has been voted up. Experience shows that you will receive a spike in traffic, sometimes lasting only minutes or hours, but that your CTR will suffer as a result since these visitors are unlikely to click on your ads. They're much more likely to hit the back button and return to digg or reddit to find their next destination.
Oh well. But it did get me listed in google' natural search results. Is that only for a few days also?
I suggest you concentrate on getting traffic from search engines, that traffic is what you need. Traffic from search engine is the most targeted traffic and is free too.