They do it via reblog (at the cost of cluttering up their beautiful, beautiful layouts). I think people also abuse the Q+A function of Tumblr too.

As I said, I'd rather have serious meta discussions on LJ. It's just easier to manage your conversations on LJ and as I said, you don't have to worry about messing up your layout while doing it. If Tumblr somehow manages to create some kind of forum creating option, they'll be rolling in money because I think that's the only thing that puts people off joining.