Facebook allows users to receive friend request from everyone and by default this Send you friend requests in set everyone. But if you do not want to receive friend request from anyone then you can disable this feature using the Facebook Privacy Settings. To change the Send you friend request setting in Facebook, click the Account drop down arrow and select privacy Settings from the available list.
This would display the following Choose Your Privacy Settings –> Connecting on Facebook
Navigate to Send you friend requests section and change Everyone to Friends of Friends. This would stop anyone from sending Facebook friends request to you. But by changing this from Everyone, you might be missing out on chances to connect with people you know.