There are quite a few long message services. Most (if not all) are plain text. Prolifi.cc allows styling via Markdown, images, etc. More of a proper article post and less wall of text.
So basically tumblr with zero sign-up?
Less than that. Tumblr has it's own followers/following/social network. This does nothing more than "attach" a blog post to a tweet. apitaru put it quite well elsewhere in the comments.