Real names were absolutely used on Usenet especially in the early days, ditto for mailing lists (and still are for that matter), even though technically they are pseudonymous. In any case pseudonymity doesn't seem like it's relevant for whether something is a social medium or not – many social media are pseudonymous (or even anonymous, like the chans). HN is pseudonymous. Reddit. Tumblr. The various Fediverse services.