"A mistake people seem to make is assuming Tumblr is a blogging platform akin to Wordpress.com, it isn't, it's a different beast entirely."

From their about page:

"What is Tumblr? Millions of people sharing the things they do, find, love, think, or create."

Even if you can do more than a simple blog with tumblr, at its base it is a blogging platform. It's perfect for a startup that is busy doing lots of other things... if it wasn't for the fact that it's always down. I hope they'll someday add a "Always Up and running, no trouble" to their "30 reasons you'll love tumblr".