Nancy Bosch's reply above had some good insights into this. Last year, we had a big push in our district for Moodle. Teachers used the forum for discussions from k-12, and I did not hear any complaints. The only issue emerges is if you want to involve (outsiders -- including parents), we do not have accounts for anyone but our students. Otherwise, if you do some research about how to ask higher order thinking questions, and prep kids maybe with a rubric (see below) about how to comment effectively, Moodle seems to do the job.
Possible Rubrics for Grading Student Comments in a Forum:
Needs Improvement: Comments seem to be completely out of place - unconnected to the topic. Little evidence suggests that the student thought deeply before commenting.
You're Getting There: While comments demonstrate some connection to the topic they demonstrate simple thinking. Reader is not convinced that the student thought deeply
Rock Solid: All comments are clearly connected to the topic. Comments demonstrate the the students
considered the text carefully and thought deeply before sharing with the
Comment Master: All comments are clearly connected to the topic. Comments are challenging and interesting to the reader -
encouraging further study of the topic and / or selection.
Adapted from: http://digitallyspeaking.pbworks.com/f/Handout_DiigoSharedAnnotatio...