It's arguable that Wordpress and Tumblr are really competitors, since Wordpress is a sophisticated content management system for publishers with an extensive library of probably thousands of plug-ins and themes to extend its functionality, while Tumblr is a minimalist rich media blogging platform of which the news feed and social features are a major attraction. Wordpress is primarily front-facing; the backend is a mess of wires and code. Tumblr is bi-directional--a Tumblr that lives on a .com makes for a pretty website that you may not be able to tell is part of a socially-driven application, but much of the activity happens inside the network's walls.