Dont be so sad, there is lots of reason for unfriend you. Maybe the people just gifted his account to other person then delete all the number, maybe that people consider to sustain his playing for a long period of time due to many other reason then emptied his friend list. I have see such people like lv 130 but didnt login in 3 months, or join the army and didtn play. They have their own reason I believe.
Take it easy.