Reminder: There's never really been any evidence that the Trump campaign's digital ad targeting on Facebook was either an important factor in the campaign or was terribly effective.
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You can't track impressions via "utm links, geofencing or a dozen other ways". Impressions are a count of how many times an ad is served to users. There's no way to know what a person does after they log-off and go about their day. There's no way to know if they even saw the ad.
I didn’t say you could track impressions that way. I’m saying that’s how you track results.