It's interesting to me that people say that they read fic in fandoms they don't know as original fic, because that's not quite how it works for me, or at least not always. When I read fic in fandoms unfamiliar to me I'm usually aware of the fact that there are nuances, character backgrounds, worldbuilding details, references etc. that I'm missing, and depending on how much the fic depends on these details and/or how obviously, the story works for me or not. When reading original fic I know that there (usually) isn't more there than what I'm reading, so I approach open questions and background details differently.

To me it seems like it would be even harder to find fic on Tumblr than on ff.net, but it probably depends on how one uses it, and on fandoms' tagging conventions etc.