They are told that they should treat the forum like a study group, so they should ask questions on the course content, reply to others' questions, share material that they found, etc. If they are smart, they realize they can have fun discussing things and prepare themselves for the tests at the same time.
Monitoring is easy.
Why, I don't know. Somehow, it is easy. It doesn't take much time to go through. I never noticed the work. In fact, it took me a year and a half before I counted the assignments and came up with 46 written assignments over 14 weeks.