There’s no single, magic solution. All of the ideas below have been shown to work. They won’t all be appropriate for you. You may need to experiment to find the perfect combination for you and your students.
Make it very clear how the forum fits into the learning aims of the course. If students sense that it is an optional extra they won’t do it. It’s like using the phrase ‘wider reading’ in an English class; you can see everyone thinking, “Brilliant, don’t have to do that, then!”
Be explicit and prescriptive about your expectations. It helps students if they know they have to make one new post and respond to two other people’s posts each week. Or if they know they are expected to write 75 words or more in a post. Make up your own rules - but do make up rules and do give them to your students.
Give discussions a clear beginning and end date. Ideally, that end point will coincide with some class work that builds on the discussion in the forum.
Give a clear goal for discussions. Maybe it will lead to an essay. Or a performance. Or an experiment. (You don’t need a separate forum for each discussion - use topic tags to keep things organised)
Make sure you and, ideally, other members of your department participate actively in the forum. Not only will this help to ‘police’ the forum but it’s important we model good practice. And, more than that, the students need to feel that their work is being recognised and valued.
But don’t respond too hastily. It can stifle discussion if the teacher is always the first to respond. Students will just look to us to provide the definitive answer.
Set it as a prep task to contribute to the forum. This might seem to go against the idea of free, open discussion, but it might be necessary, particularly at first.
Be explicit about the skills that students will develop by participating. Writing skills. Comprehension skills. Metacognitive skills. And be explicit about the fact that they will need those skills at university.
Assess the contributionsformally. You could use a rubric that is in line with the expectations you established at the start.
Set class tasks that require students to use the material in the forum. You could require them to write an essay using quotations from the forum. Or ask them to summarise the various opinions expressed in the forum.
Make it regular. You don’t have to use it constantly. It’s OK for there to be fallow periods. But you do have to use it regularly enough for it to become a habit.
Build in some exciting events. You could think about hosting a Youtube Live discussion for members of your forum, maybe with a guest expert from outside the forum. Have a quick competition - chocolate for the first person to respond to a question etc.