Google, in my opinion, made a couple of mistakes in Q1 with the way they've handled changes in Google AdSense. For example, they made the change of making only AdSense titles and urls clickable instead of the entire ad unit, saying that it will lead to fewer people clicking accidentally, raising the conversion rates for advertisers, encouraging them to spend more. While a noble cause, it certainly wasn't the wisest move to do during Q1, when ad spend is always at its lowest, especially when it will take a couple of months to catch on in advertisers' eyes. The result? Fewer clicks across the board meant fewer spend for advertisers, less revenue for publishers, and a serious decline in Google stock.
Not to let it get them down, Google just recently made a change to their algorithm in an effort to fix this mess. AdWords ads will now appear on publisher sites even when the targeted keywords aren't an exact match, instead based on overall site theme and a whole bunch of other algorithmic suggestions. The idea is that it will lead to more bidding for placement on publisher sites, and the increased competition will raise publisher revenue back to "normal". Based on my own AdSense account, so far I can say it's actually working.