One thing I noticed in the essays is how many of my students went to the class Facebook page, re-read the comments posted by their fellow students, and then cited them. In-text citations were awkward (we had never discussed how to cite a Facebook post - does MLA have a protocol for that?) but they managed to make clear where the comments came from (example: "In response to Informal Writing #3, Student X wrote..."). I was initially worried that they were relying too heavily on what their fellow students had to say but most of them used those comments to support their own views, not as a replacement for them. It also shows me that the Facebook page has become a resource for them, something they can go back to when they have a question or are struggling with an idea, which to me is a very good thing.
Facebook also came in handy this past week when my 101 class did a workshop on adding analysis and developing ideas in an essay. After working in groups, I had left a good chunk of class time for sharing group findings and allotted 15 minutes at the end for students to record their thoughts as an informal writing. The discussion was so lively that I didn't want to cut it off so I let them go and was left with not quite enough time. Facebook allowed me the flexibility to assign the informal writing as homework and ask them to post their writings online. We were able to continue our valuable discussion, they'll still get the writing practice they need, and -- as an added bonus -- they can now read each other's comments and learn from them.