When social media is new, people use it in curious ways that aren't best suited. The teacher/student friending was probably an attempt to relate to the student,

When social media is new, people use it in curious ways that aren't best suited. The teacher/student friending was probably an attempt to relate to the student, or get in contact with them for homework/etc. As my mom is a teacher, I've seen some pretty interesting build-your-own-class-website websites, and Facebook could have just been another attempt at that sort of website, for the teachers.