I don't understand the question. How would you not know who you lost, and only know you had lost someone? Wouldn't you start by knowing a specific person is missing?
Or do people actually know their exact number of friends? I guess that's it. I don't know that. I imagine I could go for years without realizing people had unfriended me.
Although it's only pseudo-related, this all reminds me of some dialogue from Oscar Wilde's The Importance of Being Ernest.
Lady Bracknell: Are your parents living?
Jack Worthing: I have lost both my parents.
Lady Bracknell: To lose one parent, Mr. Worthing, may be regarded as a misfortune; to lose both looks like carelessness.