If old friends dont' keep in touch on facebook should I keep them as facebook friends?

[Sanjay]

I have friends who I've met in the past. Some, I have spent a summer working and partying with while others I studied with at school. Now that we have our own lives and some live in different cities I try to keep in touch by posting on their facebook timeline or comment on a photo or two. However, they will only respond if I send them a direct message and maybe I will only hear from them once every year or two. I've tried unfriending people like this before and some have become upset. Are these people really still friends because we hung out years ago but now they can't take a second to interact with me?