I dunno, my earnings have grown every year (except 2009) since 2005. Last year they more than doubled, and I'm way ahead for this year.
What has dropped WAY down is CTR, and there are probably a variety of reasons for that. In my case, the EPC went up so much that it has more than covered it.
You want to look at WHAT has dropped for you - traffic? EPC? CTR? Are you getting traffic from different sources than before? Have all your pages decreased in earning, or only some? Do you know which are you best earning pages? Have you tried losing an ad unit to see if more competition for the spot drives pricing up? Does your site drive traffic to advertisers that has a good potential for conversion to sales? Are you putting your site with its ads in the view of potential clickers when they're at the exact right point in their buying cycle? Man, so many questions, so little time. The standard answer is to say Google is taking a bigger cut. Well, maybe, but until you've gone into very sharp detail over what's going on on YOUR end, I don't think that's a very useful explanation.