It doesn't seem very friendly to ongoing conversation. I mean, it's hard to reply to something that someone is saying, in such a way that other people will join in.

It caters to a different sort of audience I guess. Experience in the past tells me that most people don't want a conversation, they just want their opinion out there without counter-balancing. At the same time it would explain why a lot of people don't seem happy with Tumblr - but there's nowhere really on the internet where they've managed to get the conversation thing down. Until someone figures that out, Tumblr may be what we've got.