Discussion on facebook & google analytics differences :
1. facebook ad clicks and google analytics session are fundamentally different metrics
There is always a very strong probability that the number of facebook ad clicks do not match with the number of resulted GA session.
Facebook considered a click as a ‘hit’ (user interaction), whereas a GA session is a group of hits recorded for a user in a given time period.
Apart from all facebook track ‘clicks’ when google analytics track ‘session’
2. google analytics code did not fire on landing pages
Whenever google analytics tracking code does not fire for some reason on the landing page of a facebook ad.
GA does not track session resulted from an ad click but facebook still track and report the ad click.
GA tracking code may not fire under some situations: like code not valid, missing or did not fire because of some client/ server issue.
3. google analytics treated some of the fb traffic as direct traffic
Lot of people access fb via mobile app, and it is quite common for the mobile app not to pass refer data.
Whenever the reffer data not passed the traffic is treated as direct traffic by google analytics.
4. Fundamental Difference between facebook insight and google analytics attribution modelling
By default, google analytics attribution modelling is based on single device & single browser.
Once a user changes his device or browser analytics treats a new one
This is not the case with fb whose attribution modelling is based on multiple devices & browser.
Since your cannot use fb without login it can effectively track user across device, thus providing very robust cross-device tracking.
Other than that, fb uses the last fb ad click/ view attribution model which gives 100%, credit for the conversion to the last ad click or last ad viewed by user (whichever comes later)
It completely ignores the role of other marketing channels in conversion path.
Because of this reason the fb sales data reported by GA could be significantly different from the sales data reported by facebook.
5. Google analytics data sampling issues
Your google analytics data is only as accurate as the amount of traffic data not being sampled, if google analytics is sampling your data badly then reported metrics could be 10 to 80% of the mark.
In that case you can’t trust the data reported by google analytics and certainly can’t compare it with facebook data.
Any such comparison, can result in making marketing decisions and even losing investment.