Yes you have to do that so people recognize u but not for all social media unless ur talking bout Facebook, Twitter, kik,and I can't think of anything else. At the moment I know lots more just can't think bout it right now
That is important but to join a social network you must be of age since you must 13 or older in order to register for example Facebook. Avatar is used to represent yourself in the social community. You also must have an e-mail address in order to confirm the account and to register.
