I am friends on facebook with people I work with and those who report to me if I actually consider them a friend. So far everyone I have added I do stay in touch with if they have left the company or moved to other roles.
If I wouldn't be friends with you outside the work place I do not add them.
+1 I'm friends with lots of my coworkers and even some of my higher-ups. I also work in an industry that doesn't really care to much about what you do in your private life.