I’m sure folks here know this already, but it bears repeating. You need to give folks a reason (or multiple reasons) to visit (and continue to visit) your community. The platform you choose is only a small part of that*. The big part is the content and the tone of conversations. The people. The discussions.
Your community\forums has to differentiate itself from the facebook experience in some way… In as many ways as possible. Be compelling. Show value.
Then that content has to be published on facebook, on your pages, in the groups etc etc. Highlight members, stories etc etc.
Take some time to identify what folks don’t like about the facebook group… again both in terms of content and experience.
Focusing on quality of information and interaction for your (off facebook) community I think is the best way to go. That means that you probably won’t see mass user signups, but that’s not necessarily a bad thing.
*That said, appearance and UX is still important. There’s a base level that folks expect and they will bounce otherwise…