This subject seems to come up over and over. I'll give you the same recommendation again. Diversify. You rely on one advertising network and you are putting your income at risk. For the first 9 months of this year, Adsense contributed 42% of my average monthly advertising income (counting CPM, direct, etc.). Beginning in October, I started expanding my advertising networks and diversifying. In December, Adsense is down about 25% for me over the first 9 months of the year on average. However, my total monthly income is way up over the first 9 months of the year, and Adsense is now only about 8% of my total earnings so a 25% drop is only about a 2% hit on my total take. And some of the other networks are up this month wiping the 2% hit way. This is the value of diversification.
I rely on Kontera + Adsense. I don't have enough pageviews to get into the major ad networks. I've tried to get into Valueclick - but they've never accepted me yet. 20k daily pageviews isn't enough to get into VC?
Down a bit here too, however adding Chitika and TTZMedia have more than made up for the small dip. Problem is I wonder if I am stealing ad clicks with Chitika and TTZMedia from Adsense? Still doing pretty good this month although I will most likely be getting payments from all over the place instead of a couple places.
Based on what I have seen, earning are the lowest on December for most niches unless your site is specifically created for the Christmas season.
it is normal with adsense life but in your case i think you don't have right keywords those were clicked!
what about the traffic? does the site has the same amount of traffic it used to have? if so - it could be smarter ad management, people start learning how to use adwords better, and a lot are paying professional to manage campaigns and use the budget more wisely. Also, if your site is a niche site, it could be lots of factors from that specific industry. example - my chocolate related site always gets better paying ads around the holidays.