Hi most of us don't know why their earning fall According to Joel Comm Looks like Google focusing on long term http://www.askjoelcomm.com/why_are_adsense_revenues_falli.shtml
That's very interesting. I have to agree with him that what's good for the advertisers is actually good for the publishers in the long term. And also that the bottom line is create interesting content so people want to visit your site - too many people slap adsense on their almost contentless website and seem to expect the money to just start rolling in.
Here, here. Good comment. Too many people on this forum somehow expect Adsense to make them money when they simply slap the ads on a site that gets little traffic, has no useful content, is not focused on a niche, has poor SEO, and gets no related Adsense ads.