Within Facebook's Ads Manager, Facebook uses a 1-day view through and 28-day click-through attribution model. AppsFlyer reports only on the installs generated from clicks with a 28 day click-through.
Additionally, on Facebook's Ads Manager, insights are surfaced and based on impression or click time, not install or conversion time as we use at AppsFlyer.
It is important to remember this when comparing your reporting to Facebook Ads Manager reports.
For more information regarding possible discrepancies between Facebook and AppsFlyer, the reports are available here.