In order to use social networking sites you must sign up for them. At a minimum you will be asked for a user name and password and also to agree to a set of rules of the site. In many cases an active email account will be required.
Your online identity contains the information that others will see about you when they are using the social networking resource. It is up to you to decide how much information you include in your profile. Many users will not use their real name as their online name. Other things that can be added to your online identity are a user photograph and personal details including things such as what your favourite food is or what your favourite movie is.