Michael

HTTP includes a referrer header that is used to tell you where your traffic comes from.

So all the data is already there in our nginx web server logs for tracking the number of hits we get that originate from facebook.com (or any other domain). Awstats currently will show us this, but I don't think it breaks it down easily for reporting over time. We may need a plugin/config for Awstats. Or use another log analysis tool.