You shouldn't use someone else's social media account.
Jenny is sitting next to her friend's computer and sees that their social media site account is signed in.
Jenny wants to send someone a message on the social media site.
Jenny signs into her own account to send something over on the social media site.
Jenny uses her friend's social media account to send a message to someone.
Jenny's friend is glad Jenny didn't use their account on the social media site which would have been weird.
Jenny's friend is angry at Jenny for using their account which they see as a violation of privacy.