Facebook is like a mail server and address book, that at any random time deletes emails you got from someone (when s/he quits Facebook), and deletes that person

Facebook is like a mail server and address book, that at any random time deletes emails you got from someone (when s/he quits Facebook), and deletes that persons name and phone number. Like a phone book, that sometimes self destructs one page here and there, and then you notice some time much later. Better download & backup messages regularly, and also exchange email addresses or something else with the other person.