I would say it depends on what direction you want to take the FB group. I’m in 10 different FB groups, have joined and left dozens more, and am an active admin of one. With that being said, the majority of the time I spend on FB is in groups - so much knowledge to be shared, so much activity, so much community. It’s one of the best things to happen to me from a networking and learning perspective.
However, not all FB groups are made equally. I’m in a couple that have 10,000+ members and are solely focused on growth-hacking; they suck now.
Without question, the best FB groups I’ve been in have about 1,000-2,000 members, entry is somewhat selective, and there are active admins to ensure quality is sustained. If you can maintain the quality of the group, it is definitely the direction this community should go. Currently, FB groups > Slack communities > Telegram groups > Forum communities.