I've looked at Tumblr to stalk a few artists that I respect and my only problem is that I can't leave comments to show my admiration. Tumblr has a slightly non-intuitive UI and a damn near useless FAQ help page. Lots of art, no way to let the artist know their work is appreciated.

I guess you have to be a member or to be mutually 'followed' in order to do that because I have seen comments left by other people. It is a mystery to me thus far.