We spend $1000/mnth on FB ads. They show millions of impressions and 20k link clicks. However only 2k visits are shown by both google analytics and Mixpanel on the actual website.
It really seems like an empire that will crumble some day.
While digging into some discrepancy issues, I realised our own tracker was picking up way less clicks than what FB alleged. Across all the cases (a handful at best) I was involved in, this appeared over and over. I remember going back to our internal FB team highlighting that.
Never heard back from them and had too much on my plate to bother with it. I'm glad to see I'm not the only one that picked up on that.
Worth pointing out that if you run say cloudflare and g analytics you get similar discrepancy numbers. (Also about 1:10 in my experience).
Read various theories on why - most focus on js tracking vs pure traffic.
It often happens to me that I click on ads on my phone accidentially, but click back before the page actually loads.