If you plan to monetize your Android app with advertising, it's wise to continually test and measure which ad networks are performing well for it.
Advertising in Android apps is a frequently discussed topic, because every developer wants to maximize what they earn from their apps. Everyone is looking for one great ad network. However, while it may be desirable to find an ad network that seems good and set it on autopilot, that isn't likely to get you the most money.
The ad network that most Android developers are familiar with is AdMob, in no small part due to the fact that Google owns them. This makes AdMob the gold standard against which most ad network comparisons are made, but that doesn't mean you shouldn't test AdMob yourself.
Don't just fire and forget
Android developers are often most focused on figuring out the ad networks when they initially integrate ads into an app; unfortunately, that's not when the best information is available. As an app increases in popularity, you will see more traffic, and therefore more meaningful statistics to measure.
Despite the continual claims of every ad network that its effective cost per thousand ad impressions (eCPM) is the best ever and that all of their developers are getting rich, I typically remain cynical until I test. Even more importantly, I periodically go back and review recent performance and make changes. You may find that networks that were once performing well are no longer doing well.
Ad networks wax and wane
When reviewing ad networks for my latest book, the Android Ad Network Primer, I significantly reduced my exposure to ad networks that were not currently performing well.
For example, Mobclix has made me a lot of money in the past. However, they haven't been able to supply high-paying ads for my apps lately. They're not out of my favor for good, but I reduced the traffic that I send their way in favor of networks that are currently doing better for me.
I moved ad traffic not just for my new apps but also my existing apps to more promising ad networks that have been showing good results. MoPub has been pretty handy in helping me redirect traffic when I find some networks under-performing.
Find what is best for your app
To complicate things further, each app will have a different experience with different networks. For example, if you have one kid-friendly app and one that is aimed for adults, you might find that turning on the 'safe filter' for some ad networks reduces the high-paying ads significantly. Furthermore, with many different ad types available besides just banner ads, it takes a bit of experimentation to find out which ones generate the most revenue for each app.
Conclusion: Test and measure
It's this simple: test ad networks and measure the results on a regular basis. If you don't have enough ad traffic to justify your time to do these tests, then leave it be. If you have a lot of traffic but want better results, then you'll need to keep working to find the best ad solution for your purposes.