The group you brought together on Facebook will be totally different from the group of Discourse. At least in my experience.
The organization. Impossible thing on Facebook.
Participation. People see more topics.
Simplicity. No other tool is simpler than that.
People forget to enter the Discourse (facebook you enter every day, unlike the email).
Active participation is lower (creation of topics).
The sense of community is lower. Many people come to me and says, “Damn, I entered the forum today, but had no one.” On Facebook you add people see when they are online, chat, etc…
It’s a choice you have to do.
On Facebook you create more friendships. In my case, there were meetings, travel and couples formed. In Discourse you have better conversations, and more organized.