While you've been busy adding and interacting with friends on Facebook, you may have noticed a few friends missing from your friends list. Though you may first wonder if something's wrong with the site, Facebook notes that "friend removals are not caused by a technical error on the site" and that a friend removal requires the confirmation of one of the parties involved. Figure out what happened by examining the possible methods.
A friend missing from your Facebook friends list may be the result of unfriending on your part or on his part. This can happen unintentionally while removing other friends. If you have been removed from a friend's list, that friend will no longer appear in your friend list. Also, no notifications are sent to someone who has been unfriended on Facebook -- the only indication is that the deleting friend no longer appears on the deleted friend's friends list.
The friend missing from your Facebook friends list may have deleted her account. You may search for her by typing her name into the search field at the top of your Facebook page. If she's not included in your results, she's either deleted her account or you've been removed from her friend list, and her privacy settings prevent her account from being included in search results.
Securing Your Account
Facebook advises users who experience a sudden loss of many friends from their lists to take steps to ensure their accounts are secure. If a large number of your friends have disappeared, your account has been compromised, and you should immediately choose a new, unique and complex password for your Facebook account. For better security, change the password to any email account that is associated with your Facebook account. Never give out your Facebook password and always ensure you are logging in to Facebook's main page, facebook.com. When using a public computer, log off at the end of your session.
Sending Friend Requests
If you feel that a friend has been removed from your friends list by mistake, resend a friend request to the person. Navigate to her page and click the "Add Friend" button to send her a friend request notification. Once she accepts the friend request, she'll be added to your friend list again. You can check the status of an unaccepted friend request on the user's page. If the request is still pending, a "Friend Request Sent" message appears. If the "Add Friend" button appears, that contact has chosen to not accept your request.