Firstly, please remember that it is okay if there are small discrepancies between Facebook statistics and Google Analytics (below 15%), however if you are observing some significant discrepancies, it can be due to the following reasons:
- Google Analytics is set up incorrectly.
- You do not use UTM labels or they are installed incorrectly.
- You analyse Clicks, including not only Website Clicks, but also some others (profile, like, comment). Add Website Clicks to your analysis.
- Google and Facebook use different approaches to clicks and conversions attribution. Learn more about this: https://www.facebook.com/business/help/458681590974355
- Google measures visits whereas Facebook measures clicks, which technically are not the same.
- It is possible that Google Analytics is installed too deep and simply does not load in time to track a click/visit.
- It is possible that there is a problem with mobile traffic measurement. Try to create different labels for device targetting and desktop traffic and find out where you lose your traffic.