I plan to use Facebook groups to connect my ENA 101 course with Irwin's ENA 101 course (see Irwin's post for a description of this "Accelerated Learning Program" piloted English course). I have never used Facebook before for pedagogical purposes, in part, because I could never quite figure out how to keep the discussion and/or postings private. Irwin has assured me that the privacy of the group discussions will be maintained and the students will be familiar with the format.
Our collaborative group activity will be a targeted peer review that will address a specific stage in the writing process. There are many advantages to having students respond to the work of students from a different class. Most notably, the writing assignments will be different and the students won't be comparing their work to the writing they are critiquing. Instead, they will evaluating written work objectively, which means each paper's argument and supporting evidence will have to be explicit (i.e., for a more general audience).
I think the accessibility and familiarity of Facebook will help students feel more comfortable posting and encourage them to generate longer responses/threads to each other's writing. It will be interesting to compare the results to the kinds of discussions and responses that have been produced, for instance, on Blackboard's Discussion Board.