I met with a professor who was looking for a platform he could use with students who are working professionals getting one credit for a class through UWEC. He's thinking they'd meet one Saturday then collaborate online during winterim in more of a community of practice situation. I suggested an external network in Yammer when I heard community of practice. Yammer is a nice option because it is possible to share files and work collaboratively on "Notes" (like a wiki, or less sophisticated version of Google Docs). People can comment directly below a file, and reply to others specifically. If he needs to provide individualized feedback, that can be done via a private message. There is no grading capability in Yammer, but he said it's pretty much a "did you complete or not?" type of situation.
I prefer Yammer to Facebook for a few reasons: 1) no ads, 2) no data mining, 3) separation of professional and personal life, and 4) it doesn't change every 5 minutes.
I believe it's better than D2L for this situation because it is a more informal/less instructor led group that doesn't have strict grading criteria. He doesn't need the dropbox, quizzes, grade book, etc. It can also continue indefinitely (unless something odd happens with Yammer) and has the possibility of growing to add more members if needed. Since the students are working teachers, Yammer is a neat tool for them to learn about and possibly use on their own in the future. Yammer is also very easy to use, especially if people are used to Facebook.
He also looked into Google Plus, but I think Yammer is better because it's less of a learning curve, and you don't have to coordinate Google usernames. It's funny that I thought Google Plus was going to be the next big thing last summer, and recently I completely deleted my Google Plus account. No one uses it. Google is also pretty ad driven and data mines everything.
I'm excited to hear how it goes! Although I wouldn't recommend Yammer as an D2L replacement in a traditional class, I think this will be a great use.