Account deactivation is a temporary closure of a Facebook account. When one of your friends deactivates his account, he will still show up on your friends list -- you just won't be able to view his profile or click his name. You can remove a friend with a deactivated account from your list or you can wait until he reactivates his account; when he does, you will be able to view his profile again without sending a new friend request.
Deactivation vs Deletion
When a Facebook member deactivates his profile, all of his data is saved, from his personal profile data to everything posted on his Timeline. Your message logs with him will be saved, though you will not be able to communicate with him unless he reactivates his account. Unless his account is deleted permanently, he will also stay on your friends list unless you remove him.
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