I've previously written a blog post about Tumblr, but I've decided to revisit it after posting a comment about it on moonflowerdragon's blog tonight.
I use Tumblr in addition to my more traditional blogging platform here. I use it differently - it's more for finding, recording and sharing sources of art/craft/visual inspiration that I find on the web.

Here's the link to my Tumblr - http://sallysetsforth.tumblr.com

It's very easy to use. There's a bookmarklet that makes it really simple to send a picture, website, video or audio link to Tumblr. You can submit posts by email too, and there's an iPhone app, etc. There's also an RSS feed so that people can subscribe to your Tumblr blog through a reader, even if they're not on Tumblr themselves.

The Tumblr user interface is a bit like Twitter in that you have your own URL for your published posts, but you also have a dashboard where all the posts of the people that you follow on Tumblr appear.

You don't really comment on posts in the same way that you would on a traditional blog - you can 'like' them and/or you can reblog them and add your own comments. And others can do the same with your posts. Though, having said that, I've just discovered that you can enable 'replies' to your Tumblr posts - excellent! I've also seen others create posts with polls or question boxes attached, so there's a facility for those kind of interactions too. If you're interested in finding people on Tumblr with similar interests to you, there's a directory feature. For example, there's one for books here.

One thing that's definitely changed since my original blog post is that I have grown to love the social aspects of Tumblr. At first I found it a bit clunky in the social networking department, but once I started to find and build a network, things really changed. It's moved far beyond my original use of Tumblr as a visual bookmarking tool - it's now a place where I go to find and share inspiration, not just to record it.

Oh, and every now and then, something that you post will be reblogged by a follower with a wider reach (i.e. influence) and your post will go viral - that's very cool :)