that's the trick:

put your Forum as an app into Facebook and gain members and content.
Just need to make sure that you own the content and also know the user-data (registration data).
If you don't like FB anymore, just pull the App from FB and that's it. You still have the Forum-content (from FB users) and FB-users might still stay on your Forum and continue to use it.
Even if it's not a FB-app anymore.
Let's call it:
stealing pulling members from FB into your forum