Hi @dau
Essentially you need to persuade your members of the value proposition that a forum offers over Facebook.
As you say, it’s a tricky proposition because you’re asking people to form another habit when their lives already feel insurmountably busy. That’s why Facebook communities work so well – the habit already exists.
In your OP you list many of those value points, but from the perspective of the organisation, rather than the member.
The trick is to find out what your member’s primary values are, and appeal to those as motivators.
I manage a community for community managers and builders. We discuss this kind of stuff all day. You’re welcome to join us.