Like how closed off you and the person you are communicating with are and what your relationship is with that person. For example, we still use email to communicate with teachers. Some teachers have provided their phone number to students, but no student uses it because it might cause inconvenience or disruption to teachers. Unless there is an emergency, email is always the best way to communicate in a teacher-student relationship. I also have friends with whom I only communicate via Facebook because we are not very closed. We don't even have everyone's contact information other than Facebook. The reason for this is to keep a certain distance from people we don't know too well and to protect our privacy when communicating with them. Not just for yourself, as others may also want to keep some distance from you when they first meet. And Facebook creates a perfect platform for me