The drop is visible since couple of months, so December is nothing surprising. For me it's better, because popularity of sites is growing, but at same time, Google's pays less per click... Maybe that system works just like that: at begining you get high click rates, and after your take off, they cut more and more from our payments?
December has been the worst Adsense month for me in months - But, overall earnings are higher than ever (a record!) ... Because my earnings are diversified with things like Kontera and Premium Membership subscriptions, bad Adsense months don't sting quite as bad..
December was absolutely horrible for AdSense but I made a good deal from affiliate programs (non-adsense).
Adsense earings for me are down about 37% from November, but overall earnings are actually up a couple of percent. Diversify your advertising and don't depend on one network. When one is down, others may be up... and vice versa.