I have some FB 'friends' that I've never met, as they sought me ought via mutual acquaintances, but I enjoy swapping chat with them.

It's also interesting that people's FB personas can be so different to their real world behaviour. I accepted a friend request from someone quite senior in my profession, who I had always found dour and rather daunting face to face, but who has a wicked wit on FB.

Culling 'friends' who you want to ditch in easily done, and they don't get a message telling them what you've done.

The only group I turn down requests from on principle are my students, as that is a boundary I do not want to cross.