keyplyr - 6:53 am on Feb 6, 2013 (gmt 0)
What percentage of users use browsers that don't send referrer data at all?
All the current up-to-date major browsers can be set not to send a referrer, even my mobile web browser. Lots of misinformed people feel this is proactive security.
Don't know the exact percentage, and surely it's different for each site, but I see several dozen, maybe a couple hundred empty referrers from unique valid human visitors each day that I look for things like that. But I have no reason to keep track since I allow blank/empty referrers in my anti-hotlinking code.