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I think some of the drop off I see involves the level of participation people see outside of the class, which isnt really a condemnation of p2pu or the classes. In real world classes you can go find a study buddy or talk to someone over a cup of joe. I found that the classes I stuck with the most had the most outside of class interaction, so we could mentor earch other.
Maybe the issue is just the students dont take enough initiative in connecting and learning together. You've already identified the technical issues, however to me it seems like most of the site works pretty well,  the only improvements that need to be made are in terms of displaying classes, organizing the when/hows in some sort of easy calendar, and making some sort of homework/due date system for assignments. Also I would totally love a countdown to next class counter ;)
Speaking for myself personally in the last two weeks, I've had a particularly difficult time due to family and personal issues. Normally my days are not so stressed or hectic. I know others had to skip out on some paritcipation because of new jobs, weddings, etc. So one issue that can hardly be avoided and has nothing to do with technical or pedagogical approaches is the main fact that there IS no pressure for completion or doing things on time so it is easier to abandon assignments or the course during a period of high stress. I've been pretty motivated and active in the course so far, so for me that wasn't an issue, but I just could not devote time to it during the past two weeks. Seeing as that is one third of the total course time, it presents a major problem when looking at the course as a time-constrained class. 
A speculation might be that if the first couple meetings are frought with technical problems, or people weren't clear on what time to join in virtual conferences, they might feel those meetings are not as useful as they would hope. A tough problem, as so many virtual meetings are postponed or halted in some way because of technical difficulties. 
Perhaps more community "cheerleading" messages could be sent throughout the week to give people a little reminder about what they should be doing, as well as reassurance that there are people out there.  I think also having a platform for students to post their works-in-progress as well as a list of issues they encountered, and having those works-in-progress and issues be showcased together in  way that is easily browsable. Sometimes it's difficult and too time-consuming to slog through the threaded messages and pick the right link for the right example. (I know I ran into some problems doing my site (ie. how to make background colours for columns or something like that ), and then remembered another's students site that I saw before that had this feature and I wanted to look at it more closely... but I coudlnt find the right link and its gets a bit frustrating. Especially when there is lots of reading and lots of great exercises to do - but little time.) Phew anyway those are my thoughts for now. Hope some of it was helpful!