It's more ... I don't know, network-y? I'm not much of a Tumblr user myself, but I use Twitter and it seems like it has some similarities (except that you can *reply* on Twitter). I just don't think Tumblr's meant to be a conversation as much as "a bunch of people sharing cool things they made and/or came across with the world." (And then NOT TALKING ABOUT THEM, which is the part that drives people like you and me nuts.)