I found this fascinating quote today:
People have always been stating that Tumblr is not a blogging platform. Rather that it is the “middleground” between blogging and microblogging. I digress. Even Tumblr themselves say that it’s a nice place to blog in. If the owners themselves see it as a blogging platform why should it not be considered as one?whathappenstance.tumblr.com, whathappenstance, Jun 2009
You should read the whole article.