On sites where items are posted under user names but not necessarily proper names, enter the user name where you would normally put the author's name. Author in this case will be the main creator(s) for the item you are citing.
Creator information may often be found under a section called "About" for some types of social media, however this is not always standard.
Most items will provide a date they were posted. The standard format for the date is Year, Month Day.
If only part of the date is provided, post what you have such as the year and skip the rest of the date.
If no date is provided, use the initials n.d. where you would normally put the date.