close your facebook page then they will have to come to the board to have a conversation.
or, you could make some posts to your fb page that are on a subject that is very interesting to your board members and just post a little bit of a teaser. tell them to go to your board for the rest of the story.
have a contest for posts on the board, promote the contest on facebook.
keep the board more interesting than your facebook page.
you can't really have a conversation or discussion between many people on facebook like you can in a bulletin board/discussion board like phpbb.
remind people that posting pictures etc. is just as easy on the board as it is in facebook.