Have you ever wondered if you have remembered to "add back" all of the people who have added you as a friend?
Go to your Friend Feed. Right above the Friend Feed is a paragraph that says to "click here" to see your mutual friends. You can also see people you have added (but didn't add you) and people who added you (but you haven't added them yet).
Here's another way to get to that page - copy and paste this link into your browser: