Wow, one missed message means unfriending and no longer talking to that person? I've had messages get completely buried by others sending messages afterward. I had read the message, not been able to reply yet, and it got pushed way down in the pm queue to the point that I forgot about it. Thankfully they didn't place so much importance on a single message sent through such an impersonal social networking website. I also had a friend that I would send a message, wait, send a message, wait.......a year later, he finally got them. Come to find out, he uses his phone for facebook, and for some reason it wasn't telling him when he got messages, and he was so busy with his family and job (he has triplets, with one of them severely disabled) that he didn't realize something was wrong.
I guess if one unanswered message is enough to make you unfriend somebody, they weren't really friends to begin with.