HOWEVER, if I read through an entire discussion forum, with 150+ posts from 25 students (multiplied by 15 each week), it would take many, many hours, and unless I took detailed notes, I would have no concept of which students were stellar, average, or poor, because I couldn’t keep them straight. Now in smaller online classes that last for sixteen weeks, I DO get a feeling for who’s who, but this simply doesn’t happen in an intense six-week class like this.
Maybe I'm off base here, but I don't see the problem since that's exactly how I read these fora (although I don't officially assign grades to everyone). I don't take notes, but I have a pretty good idea who said what on which topic indicating a good understanding, who is generally a solid contributor, who has good moments in an otherwise average presence, and who needs remedial work in certain areas. For example, if I can't tell your (generic) style based on a paragraph without looking at the moniker, then you (generic) aren't participating enough. If I roll my eyes upon reading the first sentence, you (generic) are posting plenty, but usually not contributing to the conversation and that reaction happens pretty quickly for some newbies. In fact, I think having only 15 threads that are ten pages long each a week with 25 newbies to follow would make reading these fora easier. Of course, that's just my impression without any sort of experience to back it up.
To get back to your problem, Zuzu, could you skim each discussion thread and then give people impression type grades for that thread? You wouldn't need detailed notes so much as a list of names in an Excel sheet so that you could do an impression grade each post (0-5 points or even 0-3 points) by skimming and then average those grades for a overall thread grade. This would also give you an easy way to tell who is being posty with solid contributions, who is being posty without solid contributions, and who has solid contributions without being posty.
To give the feedback that you want to give for improvement, you could pick, say, two threads (based on your impression grid grade for threads, possibly varying by student, without waiting for the whole thread to be finished) on which to read for content and give comments on how to improve that could be like "You were a solid contributor on this thread because XYZ, while you were much less solid on this thread because PQR".
Again, I haven't done this for a class of mine, but I'm throwing it out there as the way that, were I to have to read these fora as part of my job instead of just hanging out here for all these hours a week voluntarily, that's what I would try.
