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:
by the end of the semester, every student is quite proud of what they accomplished - at first, they chafe under the burden of responsibility (they often think an online course is a way to hide, but in fact they are much more "visible" than in a traditional classroom), but as the semester goes on, they get really excited by what they can accomplish by making an effort with their writing each and every week.
one of my keys to success has been having the students read each other's work - there are many students who don't really care what I think (if they are low-performing students they are used to ignoring their teachers)... but the students are highly motivated by peers and by peer comments. so every week at random, they comment on the blogs of three other students in class and on the websites of three other students. this is a very important part of the class - I am the "grammar guru" and give more critical feedback about their writing, but they thrive on each other's interest and curiosity.
I keep all my materials online, and you can see the student blogs and websites here:
Mythology and Folklore
Ancient Indian Epics
I am very happy with student web publishing and would never go back to traditional writing assignments - but it did take me a few years to evolve this model which works really well for my classes!