Here is an article I wrote in 2005 about participation in online discussion. The article reports findings from a multiple case study (nine online classes), looking at how the discussion activities were designed, facilitated, and graded, and what happened in terms of quality, quantity, nature, and timing of student posts.

Dennen, V. P. (2005). From message posting to learning dialogues: Factors affecting learner participation in asynchronous discussion. Distance Education, 26(1), 125-146.

TL;DR —

• Students will follow whatever guidelines you give them, so give specific guidelines.
• Deadlines drive interaction for most students. Incremental deadlines can help support interaction.
• If the instructor participates too much, the discussion becomes instructor-centered and students won’t interact with each other.
• If the instructor is absent, with no clear indicators that they are even reading the discussion, students may get off topic, stop participating, and even talk about the instructor in the third person.
• Students want feedback on their discuss performance. It can occur in various ways: privately when grading, en masse through announcements or messages to the class, or directly through instructor interaction on the discussion forum.