I have been looking into social networking and bookmarking sites. Tumblr kept appearing in lists and posts about the subject. So I joined it to try it out.
It is excellent. Even though it's known as a "micro-blogging" site (similar to Twitter) it's actually a lot more versatile than the label implies. Unlike Twitter, you're not limited to very short posts. And it actually has as much functionality as most other blogging platforms, if not more.
It is similar to Twitter in that you can follow other members of the site, and have your posts "re-blogged" which is like having them "re-tweeted". So, if you've got a knack for writing stuff that attracts this kind of treatment you could end up with a lot of traffic pretty quickly.
The various site templates you can use are much more unique and creative than those you'll find elsewhere. Which is why there seem to be a lot of arty types (as opposed to newsy, pundit types) using the site.
One attractive aspect is that you can use your own custom domain for your site at no charge as well. Not many blogging platforms offer that.