This is kind of interesting. I suspected as much but its interesting to look at numbers. I have reference links but I assume I should not post them.
Total online advertising revenue (billions)
2013 - 117.2
2014 - 133
2015 - 149.2
Google network sites revenue (billions)
2013 - 13.65
2014 - 14.54
2015 - 15.03
Google Share
2013 - 11.65
2014 - 10.93 (6.2 percent drop)
2015 - 10.07 (7.9 percent drop)
I don't know what people would attribute this too. My guess is google is still pretty much the main game in town for small publishers. But for mid size publishers (50-200k pageviews a day) there are a lot of other attractive options. And other ad networks are starting to equal google on revenue and far surpass them in terms of customer service. And it seems some advertisers are moving to other networks.
Google revenue from network sites has started to slow down. (For comparison from 2009 to 2010 it went from 7.17 to 8.79). The next question is if it will start to fall eventually.