Hello,
there is an inherent technical difficulty in tracking downloads and differentiating them from superflous / repeated "touching" by web clients and other applications. E.g. a PDF reader may download a PDF file progressively which results in multiple "partial" HTTP download requests.
The simples solution would be to modify the "Download" action in Share to manually trigger some kind of notification. You may get a lot of false positives if users decide to cancel the download. Additionally, you might want to think about modifying "Show in Browser" too, which could effectively be considered a download as well.
Regards
Axel