I referred a friend but I haven't received the reward. What's wrong?
Unfortunately you will not get the bonus if the game detects that your friend has been using the same internet connection as you. This is to prevent people from simply making lots of new characters themselves and getting all the money. Therefore, if you refer a friend, make sure you don't both try to play from the same computer.
Further more, your friend must have cookies enabled in their browser when they click the referral link, and must not delete those cookies before they reach level 10. This is because the game uses a cookie to track the referral - without it, it doesn't work.