Why doesn't insights match the graph?
I have seen many instances where the Likes, Comments, and Shares reported from the Graph API endpoint for a particular post do not match the stats reported by Insights. I.e.:
likes 1000, comments 450, shares 300
likes 725, comments 375, shares, 200
Or something like that. I do realize that sometimes, insights will show higher numbers because Insights tracks stories that are generated off of a shared copy of the original post (very cool), but these are cases where Insights reports numbers lower than what the Graph API endpoint shows.
Is this because after 28 days, Facebook Insights stops updating for individual post stats?
I've filed a bug here: https://developers.facebook.com/bugs/400094966692945
The unfortunate reality is that Facebook Insights is horribly buggy, and in general fixing api bugs does not seem to be much of a priority for Facebook.
If you visit your Insights page frequently you will see this. Days where the count of app removals spikes by 1000%, followed two days later by user installs spiking 1000%. Days where there are zero comments on your app's posts where you have recorded thousands in your database. The list goes on...
Even when they "fix" Insights bugs, they don't always fix the history, leaving the inaccurate data in place and only correcting it on a go-forward basis.
This is the kiss of death for any reporting or analytics system. Once you start seeing the bugs over and over where you know from your own independent data that the reporting system has bugs, you start to question everything it shows you: is this reality or is it a bug? How can I make any decisions based on this data if I cannot trust the data? Folks with enterprise experience know this, but the kids who run Facebook...not so much.