To be fair, I'm using Discord to replace Skype and before it AIM much more than tumblr - the conversations I'm having that way wouldn't have been public before. But I don't know the extent to which other people are doing simila, or not.

And yeah, I think some kind of public fandom directory of channels - not necesarily anywhere close to all of them, but enough to reach a decent mass of options - would be necessary for Discord to really become a fandom hub.