However, I've found another great reason to use a discussion forum: differentiation. As a 9th grade teacher, I don't want to label any of my students for fear that they will define themselves with that label, so I struggle with the idea of ability grouping. After all, my students can figure out pretty easily where they fall, so I've taken to secret ability grouping via discussion forums.
For our last novel, I used three leveled questions but never labeled them according to the level. I merely assigned the groups by the title of the question (i.e., conflict, character, point of view, theme, etc.) and the students responded according to those groups. I don't think they ever made the connection that it was ability grouping either, so it was a successful endeavor. I was even able to give students more individualized feedback and push them in the ways they needed to be pushed to grow and learn. It might take more time for me, but I think it is worth it to hear from each student, too. So often our quiet students don't speak up unless asked to, so it was a nice change of pace to hear from everyone and monitor their individual progress. After all, there's no hiding in a discussion forum.
If you're struggling to get students to participate or to differentiate in a meaningful way, I would recommend using a discussion forum. It might take the students a couple of tries to get the hang of it, but like the blogging, mine have found it useful since it breaks our studies down into more manageable pieces. I've even seen some of their discussion post ideas coming up in their final essays, so that is more evidence that the discussion forums work.
Reference: Morrison, G. R., Ross, S. M., Kalman, H. K., & Kemp, J. E. (2011). Designing effective instruction (6th ed.). Hoboken, NJ: John Wiley & Sons, Inc.