While digging into their data, we realized that since Wikimedia started the HTTPS switchover and an increasing portion of inbound traffic happens over SSL, Wikimedia sites may have stopped advertising themselves as sources of referred traffic to external sites. While this is a literal implication of HTTPS, it means that Wikimedia's impact on traffic directed to other sites is becoming largely invisible and Wikimedia might be turning into a large source of dark traffic.
I wrote a proposal reviewing the CrossRef use case and discussing how other top web properties deal with this issue by adopting a so-called "Referrer Policy”:
Feedback is welcome on the talk page:
Dario