I joined Tumblr because a number of my fannish friends were there, then followed a few more that I found interesting and/or pretty, and then it became a big time sink without a lot of return and I stopped going. I don't like the difficulties the structure creates in engaging and discussing, either--it feels more like a lot of individual soapboxes set up around a town square, rather than a room full of people talking to each other, to me. The comment structure fractures discussions and makes them hard to follow beyond a single tree. It's pretty but annoying, and it skews very young, from what I've seen. It does seem to be fandom's future, though, that and AO3.