I’ve been discussing with the folks at CrossRef (the largest registry of Digital Object
Identifiers, think of it as the ICANN of science) how to accurately measure the impact of
traffic driven from Wikipedia/Wikimedia to scholarly resources.
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: