Maybe not rich but there are several plugin developers that make a decent amount of revenue by selling plugins. For example check out Dan Grossman and his review plugin: http://wpreviewsite.com
the real question now becomes where is the information WA is getting its results from and how is that chosen as the premier source for a particular query?
I think the differentiating point is that Tumblr is not an adult website. Adult websites need to stay separate from those which are popular for kids <18 years old or at least have some kind of warning that informs the user about the content. We have the same thing for TV, music, movies, video, and video games too.