As you would imagine, Google have said that their share of the revenue is used for maintaining and developing the AdSense program. You can read the full post from Google on their Inside AdSense blog.

Google has been pressured to release these figures for a number of years now, and after some developments earlier this month, they have decided now is the right time. Its also important to note that the figures provided above do not include larger publishers that work directly with Google. My guess is that these sites may see even higher figures.

To be honest I think that a 68% share for the majority of publishers is quite fair and a bit higher than I was expecting.