Fun....has the course covenor said anything anywhere about what the criteria are? If not, I think I'd make half the mark for participation e.g. how many substantive comments did each student make (unless they all had to post one comment each week or something) and half for the content. Do you have to mark them each week or just give an overall mark / set of comments? If it's weekly if you can somehow mark them on-line, you could set up a document with standard comments and cut and paste them in. I once had to mark weekly commentaries on 'what they'd learnt that week' from the course and found it very hard to come up with anything much to say, so you have my sympathy if it is weekly.