We recommend that all advertisers use Facebook’s SDK to measure mobile installs. The developer needs to install the latest SDK and set up their app to measure the performance of your mobile ads:
However, if your client is currently using a third-party to measure installs or would prefer to work with a third-party (i.e., for lifetime value calculation), we have partnered with companies to provide measurement solutions for mobile app install ads:
You can confirm that the volume of installs is being correctly reported in the Insights section for your app in the App Dashboard. In addition, in the App Summary Page, there is a time stamp 'Last Mobile Install Reported' for iOS and Android so that you can confirm that an install is being reported.