[kordless]

Or. Hits to a website result in a list of unique IPs accessing the site. Remove any IPs from the list which are known to be running JavaScript. The remaining IPs aren't running JavaScript or are ignoring/breaking your script.

This begs the question whether or not there was a scenario where there was a hit to the site, the referrer was set to Facebook, JS was enabled in the browser, but the resulting hit didn't result in a JS enabled request to the analytics software.

Another poster mentioned prefetching as a likely culprit. Google is known to prefetch search results for you, but it requires the use of the rel="prerender" tag. I find it highly unlikely that is what's happening here.