People invest in hot properties like Tumblr for the same reason people will pay a significant premium for real estate on busy intersections: they have access to

People invest in hot properties like Tumblr for the same reason people will pay a significant premium for real estate on busy intersections: they have access to tons of "eyeballs" (I know, I know, hackneyed term) and it's a lot easier to figure out how to make $85 million with 10 billion page views then it is to start with $85 million and 0 page views.