Writing for Knol is a joy and a scourge. The content management at one time was the best on the net. Now, a lot of features are disabled or broken. It's hard to simply maintain existing work, much less create media-rich new work.
Earnings? You gotta be kidding. AdSense revenue requires far more clicks than you are likely to earn at Knol. Most of the original highly rated writers (I'm rated #1) have stopped producing new work because Google has utterly failed to maintain the platform.
A strong cadre of top-rated writers is unanimous about Knol: Google is foolish to permit the current decay. In my opinion, it is corporate malfeasance. Google does not even enforce their own Terms of Service or Content Policy. The site is sinking in spam even as legitimate academic and scientific groups consider participation.