I've tried now, in two classes (one at an online university, the other at an online high school), to use wikis and collaborative writing as part of my formative assessment. The online university was asynchronous, the highschool was synchronous. Both did not work out very well, but for very different reasons.
I think it was James Paul Gee who coined the phrase 'digital natives', ie, our students are immersed in digital media, they understand it intuitively, and we, as 'digital immigrants', will never wade through the sea of 1s and 0s as successfully as they do. At the online high school, this did not prove to be the case. My students were certainly familiar with digital technology, but their familiarity was profoundly superficial (if I can say that). While they had often used wikis (most often as a source of information for History class), they had never actually considered what was involved in making them, or the implications of how that information was collated/created into the wiki article they so freely copied. I had to take them by the hand (voip-style) and talk them through the entire process several times before they started to catch on... but by that time, the needs of the curriculum were such that I had to abandon the project.
At the online university, my course was organised by topic, over the duration of the session. The student on their own time, whenever they felt like logging in, could digest whatever topic they chose. At the end of each topic was a wiki, with several possible article suggestions. The idea was that instead of writing a 3000 word term paper, they'd write - and edit others' - short articles (which in total would equal more or less the same amount of work). In the process, they'd be communicating with each other via an online forum or by voip, and as a class they'd create what would be essentially a text-book. It's now the end of the session, and I finally got the first wiki articles posted, all in a rush. No time for editing, no time for collaboration, just a series of v.small essays, with no external links or images.
Clearly, I hadn't explained the concept well. Just as clearly, I can't rely on my students to be motivated enough to get the articles done early enough so that the collaboration process can start. I dug a little, and found that the main problem, as far as my students were concerned, was the fear of letting others see their written work. Procrastination was of course another issue, which was compounded by my error in letting the students 'choose their own adventure' through my materials. And finally, like my high school students, they were not familiar with how/why wikis work (which I found astonishing - but why should I? I spend every day online and so encounter wikis all the time, but my students apparently do have lives... :)
The next time I run that course I think what I shall do is abandon the wiki format in favour of journals (that only the student and myself may view). The topics and questions and the amount of writing will all be the same. I will also set firm deadlines and chart a linear progression through the course. And I think I shall make the students watch the wee video below: