I suspect the search engines (Google, Bing, DDG) could do a huge amount to combat this if they so wished.
Just check if the HTTP and HTTPS site returns near identical results (e.g. same title, response code, etc) then link to the HTTPS version by default.
A lot of links that get posted are directly taken (ultimately) from search engine results. Someone will search, view the page, then copy/paste it to others.