extracting money from mobile
is a constantly changing business. Within a year there has been a dramatic shift from people paying for apps to in-app purchases and now people are getting wise to that, apps are shifting to advertising.
As an app developer, the variation i hear about in techniques of what advert types and where is most effective is huge.
Companies like google do well in advertising because they know how to get quality advertisers and how to execute advertising well (in terms of driving conversions).
Of course its not good that it is dominated by one player, but as is said of other things; you wont get fired for choosing google for your advertising.