My CPC and revenue dropped starting on January 2nd. December was a great month as usual, and it always slows down right after. Looking at my stats from last year it was lower revenue in Jan and Feb, so I expect the same thing to happen again this year.
I have seen that there are some months during the year that the CPC drops in some niches, since that is based on the bids of the people paying for the ads. It could also be the niche as a whole if there are few advertisers. For example one of my keyword's CPC went from $1 to 0.60