Over the last few months, I noticed a steady decline in AdSense earnings. Checking the statistics revealed, that this was neither due to decreasing traffic nor a lower clickrate. Both, traffic and clickrate, have been fairly stable. What went down instead, was the earnings per click.
After looking through my website, I came across a couple of well-visited pages, featuring ads for free smiley downloads. Ok, culprit found. Someone, who offers things for free cannot afford to pay a lot per click.