You can create non-paid ( also known as organic) Facebook posts that include links to the iTunes Store or Google Play Store to download your app.
Each store handles this differently. The Google Play Store allows apps to identify devices that click on a specific URL through the use of so-called Android referrer parameters. If this information is captured by your app (i.e. if your app reports Android referrer parameters to the Localytics SDK and if you've turned "Android Referrer Mode" on in the Localytics Attribution Settings page), then this information will be handed off to Localytics so that we can expose this in our reporting.
iOS does not support this methodology.
One possible alternative for tracking organic Facebook-acquired users is to use device fingerprinting.