There are be several reasons that hosting a forum/fanclub on Reddit is better than on Facebook. Facebook mandates real name use, one username for everything that not everyone is comfortable with, I'm sure everyone on HN is already aware of that. But I think there is another subtle reason.
In the past, I have relied on Facebook to host another hobbyist project, and it was super active. I thought what is more convenient than having one place to notify users about the ongoings and stuff and plus having a mobile app that kicks ass, right? It turned out, after a while, the group died. No one would interact with our news anymore, even when we don't spam them with junk news. I think the subtle reasons is that Facebook is way too aggressive on interaction. Maybe something exciting is happening and many people flock to comment on it, and so Facebook spams everyone with notifications and emails about the current ongoing that many of the users don't care about. If a user gets annoyed and decides to mute the group, then I will lose a good, caring person forever. Reddit has no such jealous nudging mechanism, the user has to explicitly check that subreddit to know. So the participation becomes much more passive and the user can choose to update about the ongoings at their own convenience.
It turns out being more gentle for hobbyist groups wins over being aggressive. Not that I only have those two choices, though...