Now that you mention it, as of today I am -26% from last month at this time according to my adsense page.
So it's typical for Jan to be lower than Dec. My guess is that advertisers are paying higher bucks in Dec.
Although my calculation shows
Dec 2011 Jan 2012 -15.01%
Dec 2010 Jan 2011 - 48.63%
Dec 2009 Jan 2010 - 17.55%
Dec 2008 Jan 2009 -34.38%
Dec 2007 Jan 2008 -22.79%
Generally speaking, ad revenue craters after Jan 1, then slowly recovers as the month goes on. Advertisers buy their campaigns on monthly/quarterly/etc basis. December is usually a 'big' month, then those campaigns all expire after Christmas.
That said, AdSense is a black box. CPM jumps sometimes for (seemingly) no reason, or craters unexpectedly. Other ad networks I've used are much more clearly affected by seasonal variations in the ad markets.
Nope, ours went up - even had a spike upto £90 for one day!
Didn't get anywhere near that for the rest of the month though, still had the best month so far though. I've found the income pretty steady really. But I'd imagine it depends on the adverts shown on your site, more competition = more money for adsense.
Perhaps - I don't know, I just know that I wish I'd put adsense on a while ago. One very, very interesting thing I did last week after I found out you could do it - add a custom variable into your Google Analytics code. I've put a bit of code in that finds out if the visitor is a Guest, a Member or a Supporter (i.e. paid member) and stores this in the Analytics feed.
Now in Analytics I can get stats for each of the 3 user groups - including Adsense revenue figures!
Very eye opening and indentifies where you get most of your revenue from. In my case, Guests and shows that I almost certainly could drop the Adsense for supporters and lose only a few cents in revenue. I'm going to leave it going for a few weeks to get some decent data before I do it though.