'All students use Facebook all the time so why not use it for teaching and learning?'
This is not true; not all students use Facebook and among those that do many use it only for social purposes. They many not wish to interact with their entire cohort or lecturers there, and may struggle to deal with the resulting overlap. Ask (rather than assume) what your students would prefer.
Further reading: 'Get out of MySpace' journal article in 'Computers & Education' written by staff the University of Glamorgan (as was). The URL for the article is http://www.sciencedirect.com/science/article/pii/S0360131509001857