Facebook: You Sure You Know Him?
Facebook users were allowed to send friend requests to anyone they want. This resulted in some users misusing the ability by spamming others with unknown friend requests. Later on, Facebook imposed a limit on the number of awaiting friend request a user could have. Once a user reached that limit, either they had to cancel an already sent request or the other user had to reject it, before they could send a new one.
From the image below it appears Facebook is once again changing this system.