There were a couple links here today in the "New" section with headlines that peaked my interests enough to click on them but they went to "Medium.com" which requires I "sign in" with Google or Facebook.
I won't do that. I'm not a hardcore "do not track" surfer but I have my limits and that's just a line I won't cross.
I don't think it's a leap to say this probably happens more than authors think.
As for Medium, it has a lot of people using it because it was popular, fashionable, and free for a long time. These days as a reader it is terrible - I don't bother clicking medium links, and as an author I'd want to host my own content, not get it mixed in with the other noise hosted nearby.