A Facebook miért kéri a mobilszámomat?

és ha pl. nincs telefonom? (van, de mi lenne, ha nem lenne?)

ezt írja:

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Használj telefont a fiókod ellenőrzéséhez

The phone number you use can only verify one account. Once you enter your number, you'll receive a code that you'll be able to enter on Facebook to verify your account. Your phone number will only be used to verify your account and will not be shared with anyone without your consent.