The same thing has been true of mobile games for a while. A lot of the ad inventory displayed in free games is ads for more free games -- sure, there's some real revenue from in-app purchases, but there's also a lot of money getting passed from hand to hand for ads.
I think that 'some' real revenue probably dwarves most industries tenfold. Look at what something like Candy Crush makes in IAP, the top 5 players could probably support the entire ad industry themselves in perpetuity without breaking stride.
It was ridiculous. There was no real money, just app developers paying each other for installs.