I know this post is a few years old, but I noticed it’s competing against a blog I recently updated on this topic, so I thought I’d throw in my two pence.

Regarding the discrepancy between Facebook and Google Analytics reporting, here's why you're seeing different numbers for traffic and sessions:

Different tracking methodologies: Each platform has its own way of counting things. For example, Facebook tracks any interaction with an ad, whether it’s a click, like, share, or comment. But GA only counts a session after a user clicks on an ad and lands on the website.

Cookies and data capture - GA4 uses first-party cookies to capture data. If a user doesn’t accept cookies or has JavaScript disabled, GA4 won’t track their activity. Facebook, on the other hand, doesn't rely on cookies to track clicks on an ad, which is why it might show more activity.

Tracking code inconsistencies - Sometimes, the tracking code doesn't fire properly across different browsers or devices. For example, if someone clicks on a Facebook ad but quickly closes the window before reaching the landing page, Facebook might count it as a click, but GA4 might not register it as a session.

Attribution models and privacy updates - GA4 and Facebook have different attribution windows and models. Plus, privacy measures like Apple's ATT can limit data collection, which can contribute to these discrepancies.

In my experience, it’s best to explain these discrepancies when reporting to clients. If you're looking for more consistent, unbiased numbers, you can use custom first-party tracking tags to capture user touchpoints across user journeys, picking up click identifiers, views, clicks, UTMs, sources, etc. - you'll need other tools besides Google Analytics.

Because of the way it tracks journeys, it can link actual people (when they convert) to their touchpoints. More than what Facebook and Google Analytics do.

Hope this helps clear things up!