Thinking about some of my grade school and high school friends who have become teachers, they pretty much all have Facebook accounts, and they pretty much all have pictures/information on their Facebooks that could be deemed inappropriate if their students/their students' parents/their school administrators picked over it with a fine-tooth comb. Photos from nights of drunken debauchery featuring scantily-clad guests are almost universally inappropriate for the workforce (unless you work in the entertainment/adult industry), but even the groups someone is a member of Facebook can be seen as offensive to some people (i.e. groups that reveal one's political affiliation, stance on hot button issues, etc.). I don't know how much discretion my FB friends who are now in the workforce use when they consider what information should be on their Facebooks, and how selective they are in blocking certain groups from seeing certain info. Facebook is a really useful tool for communicating with all sorts of people, but you have to be really careful that what you think is 'private' on Facebook is really private.
OP, it seems like your former student hasn't matured at all since his antics in your classroom a few years ago. I wouldn't suggest having anything to do with him on Facebook.