Apple iAd versus competition
The loss of market share for iAd is probably not as dire as people are making it out to be. Apple set up an easy way for iPhone developers to put ads in their apps without any work at all on their part, which is great for small developers. Big developers can afford doing some work on their own in exchange for keeping all the revenue.
iAd is therefore limited in three ways:
1) by Apple's market share (since iAd is iPhone only)
2) by the percentage of apps that have any advertising at all
3) by the percentage of apps made by "smaller" developers for whom it isn't worth the hassle of doing anything beyond enabling iAd
Apple's market share (in the smartphone market, not the overall phone market) will decline over time since smartphones will be 100% of all phones sold in a few years, and the bulk will be super cheap Android phones replacing the feature phone category. This is already happening, but losing smartphone market share to Android doesn't mean Apple can't still double iPhone sales YoY for a while yet.
This drop in smartphone market share for iPhone is the cause of the drop in iAd share. It isn't that iAd isn't serving more ads and making more money YoY, just like iPhone sales, its just that the various ad networks that serve Android are growing faster due to Android sales increasing at an even faster rate than iPhone sales.
The reduction in campaign sizes is probably not also something to be too concerned about. This brings more potential advertisers, since there are many more potential clients with a $100K budget for online ads than clients with $1 million budgets. Using a very large campaign size at first made it easier to get this off the ground, since they've never done this before and probably didn't want to be flooded by 50,000 potential clients. They'll have to continue to decrease it as they (and everyone else) moves towards the holy grail of location based advertising.