Earlier when I wrote the post, this feature was not yet implemented. But now that’s not the case, as there are two ways to cancel a friend request / invite which you send to people who you don’t know or misplace by mistake:
1. Login to your FaceBook account and navigate to your ‘Privacy settings’. Find the person you’ve sent the request to by mistake and block that person. Immediately unblock the person. When you do this you’ll notice that the request is off ! This is a crude way to get it done.

However, there is another way to achieve the same:
2.Login to your FaceBook account and navigate to your friends list. There in the view, you’ll notice that your friends are nicely segregated to different lists. In one part, you’ll find the list of people, who you’ve sent a request to, and haven’t heard anything from them yet. It would say ‘awaiting confirmation’,  next to their name.  Click the ‘close’ or ‘X’ button and that will take care of the unanswered request.