People seem to take it personally when someone they don't talk to anymore deletes them off the internet. I could understand if someone you truly cared about didn't want to know you anymore, or had a lack of respect for you. But a person that couldn't care less about you? The question people should ask themselves is "Is there any benefit keeping this person on the internet?" If not, then what's there to lose? There's plenty of people in the world - I'm sure that are more worth while keeping in contact with, rather than someone you'd classify a "stranger."
If you're the one doing the deleting, it's nice to clean out the useless people to allow the worthwhile ones to see your profile. Keeping people on the Internet because it's "funny" while having a giggle at the stupid statuses and photos may degrade themselves with is really a form a bullying. Not to mention, naive and something teenagers do. If you don't contact or care about them, there's an unfriend button right there on their profile page, all people need to do is click it.