WordPress, which opened for business in 2005, is going great! It hosts 19.9 million blogs, according to Royal Pingdom, and has been millions ahead of Tumblr–but the gap is closing quickly. Tumblr now says it hosts 18.4 million blogs–they just hit 17 million a few weeks ago–and is growing at an alarming pace, hence all the downtime over the past year.
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.