How to verify that installs *are indeed measurable for your App by Facebook:
a) Go to the Facebook dashboard and select the appropriate app in the left menu
b) Go to Insights and click "See All"
c) Select "Mobile App Installs" in the left menu
If Mobile App Installs are more than 0, then you should be enabled for the CPI pricing model
How to enable CPI?
To enable Facebook to count your app installs you must
1) register your App with Facebook (select "+Create New App" here)
2) Then you must enable install measurement in one of 2 ways:
Integrate with the latest Facebook mobile SDK (iOS and Android)
Integrate with any of the Facebook mobile measurement partners