How are they going to prove you are in control of a social media account? What do they do if you tell them you don't remember any of your passwords, in fact you

How are they going to prove you are in control of a social media account? What do they do if you tell them you don't remember any of your passwords, in fact you wrote them down on paper because you were that bad at remembering them and just happened to lose that paper? What happens if someone does not use social media, are they going to deny them on the basis that they don't use Twitter or Facebook?