Why should I be friends on Facebook with people who were mean to me in school?

I find myself being asked to be friends with people from school or scouting 40 years ago, who were quite abusive, or wouldn’t give me the time of day.
I want to be a good, forgiving Christian, but I am annoyed by the “let’s be friends!” attitude of some people who made life miserable so long ago.
Any suggestions or guidance?