If you have the option of moving people off of facebook, how about a slack or discord group?

If they won't move off of facebook, I'm not sure there's anything you can do to retain the same level of interaction. Maybe you could allow yourself a reduced level of interaction while still feeling connected. For example, an SMS every couple of days should be plenty enough contact to keep up with any significant events. If you really want to take the reins, you could organise events yourself, ensuring you won't miss them.