When one develops a "crush" or extreme liking to someone's Tumblr page and imagines this person would be their soulmate in real life.
"Meaghan has a huge Tumblr Crush on OMGREG."
A crush on someone you've met on Tumblr or a crush on their blog.
Anne: I have a Tumblr crush on someone I've met online.
Billy: I have a Tumblr crush on your blog.