Tumblr hosted the early-2010s origin of the SuperWhoLock fandom crossover.
SuperWhoLock was a fan-made crossover joining Supernatural, Doctor Who, and Sherlock. It emerged through Tumblr’s highly shareable format, where users could reblog posts, add tags, combine fandoms, and circulate fan fiction, artwork, edits, and animated GIFs rapidly across overlapping communities.
The crossover’s history is more specific than simply saying it was “popular online.” Media scholar Paul Booth located an early GIF fic dated December 30, 2011, and the term had apparently been coined before that. A fan-made SuperWhoLock trailer in April 2012 also helped the idea reach a much larger audience.
Twitter, LiveJournal, and Reddit were important internet platforms, but they are not the canonical answer to this question. LiveJournal was especially significant in earlier fan communities, while Reddit later helped circulate many memes; SuperWhoLock’s defining early home and cultural identity were tied to Tumblr.
The fandom was less a shared official universe than a meeting point between three separate fan cultures. That distinction explains why its content often focused on imagined character reactions and mash-ups rather than continuity between the television series.