Is it appropriate to clear "my Facebook friend list" every month?

[S. E.]

For the past two years, at the beginning of every three months, I go over my list of Facebook friends and delete anyone that I haven't talked to for a year or so.. The reason I've been doing this is because I don't think that there is a purpose for adding someone as a friend to your Facebook account if he/she never writes in your wall, messages you, etc.. Is my decision considered to be appropriate? Do most people "remove" friends occasionally? Will removing friends "isolate" me from others? I have around 200+ friends as of now.. I would have probably 500+ friends if I haven't deleted some friends from my list.. How do you base your decisions when you add/ignore/delete friends?