They never promise or make it transparent, although in an earnings call in Q1 2007 they basically said they only keep 25% (they generated $1.2B and paid out $91

They never promise or make it transparent, although in an earnings call in Q1 2007 they basically said they only keep 25% (they generated $1.2B and paid out $916m). I have noticed that this year my AdSense revenue has fallen even though click-thru rate and impressions have been constant. I assumed it was lower spend on the advertising side, but a bigger Google cut may make sense.