I only keep current friends on there. I've deleted most of the people I went to school with, bar one or two, and all my relatives.:D
My brother attacked me there one day after I challenged him on a thread he started. I wasn't being negative, just pointing out some integral information about the topic, and he just lost it and started telling me I was 'one f*cked up bitch" and how could I dare think that dad might be a pedophile, and blah blah blah...I deleted him without hesitation, and then he started texting me to continue the rant and call me petty for deleting him!
Some people are just unbelievable.
Most of the people I have on there I really enjoy communicating with, and if I haven't heard from someone in a while, or they never contact me or interact with me, than I will delete them. If they piss me off even the slightest by being condescending or such, I delete them.