The most important thing to know about Facebook as a learning tool is what it isn't. Facebook isn't Blackboard or any other course management system. It isn't a wiki, or a blog, or any sort of silver bullet tool. Facebook is the digital social center of the college campus. It is a social tool; its use is primarily the management of the social life at college. Of course, college life is geared around academics, so inherently the social worlds of college students intersect with academics - but only to a certain extent. Knowing Facebook's limitations and target uses - they are primarily social - will help you contextualize your use of Facebook as a learning tool.
The expectation of privacy. Facebook has unique boundaries when it comes to privacy. Students know that staff and instructors are on the Facebook, but they primarily expect that their profiles will be viewed by their peers. Therefore, you must respect (at least in name) student privacy in Facebook. What does this mean? You must let your students friend you. You must not cross privacy contexts first. You must reciprocate disclosure and be an equal player. I will explore all of these in greater depth.
Non-participants. A good deal of Facebook users do not allow faculty, staff or other persons of power to be part of their social life. Therefore, there should be no expectation that Facebook will allow engagement with all students. We can only respect the student's decision in this matter.