What you’re fighting here isn’t the group, it’s ease of access. There is already a Facebook app installed on most phones, everyone’s already on Facebook and checking it multiple times a day, it’s right there and they can get their information easily. We’ve had to do a “if you can beat them, join them” approach for our nonprofit. We encourage forum use for a lot of things, but run a 25k Facebook group to meet people where they are. You’re fighting psychology, not logic.
If you want people in the forums, you need to do things in the forums that people want to see and can’t get anywhere else. Make sure you have Facebook login enabled so it’s super easy to just hop on over with as little friction as possible. Are there plugins for integrating Facebook content in some way? (I doubt it since FB really doesn’t want people to leave the platform, but it doesn’t hurt to check.)